Update 5.55pm: The Irish flag to be flown at half-mast tomorrow and Thursday "out of respect for those who lost their lives in Brussels and following the tragedy in Buncrana".
Update 5pm: Senior United Nations officials have strongly condemned today's terrorist bombings in Brussels, extending condolences to the victims and their families while expressing solidarity with the people and Government of Belgium.
A statement issued by Secretary-General Ban Ki-moon's spokesperson said “the despicable attacks today struck at the heart of Belgium and the centre of the European Union.”
“The Secretary-General hopes those responsible will be swiftly brought to justice. He is confident that Belgium's and Europe's commitment to human rights, democracy and peaceful coexistence will continue to be the true and lasting response to the hatred and violence of which they became a victim today,” it added.
Also reacting to the terror attack, the President of the UN General Assembly, Mogens Lykketoft, said he is “horrified.”
“We have in the last week seen atrocities in Turkey, Ivory Coast and now in Belgium. It must be condemned in the strongest terms,” Mr. Lykketoft said in a statement.
“Acts of terrorism are unjustifiable regardless of their motivation and terrorism in all its forms and manifestations constitutes on of the most serious threats to international peace and security. Acts of terrorism have no place in the modern world and only serve to strengthen the resolve of governments the world over to find and prosecute the individuals responsible,” he added.
Meanwhile, the United Nations World Tourism Organization (UNWTO) said it is “deeply shocked by the tragic attacks” perpetrated today.
“This is not an attack on Belgium, it is an attack on us all and sadly these tragic events remind us again that we are facing a global threat that needs to be addressed globally,” said UNWTO Secretary-General, Taleb Rifai.
Update 1.40pm: The National Security Council will meet this afternoon to review Ireland's security arrangements following the Brussels attacks.
The council includes the Garda Commissioner, the Chief of Staff of the Defence Forces, and the heads of four government departments.
Update 1.25pm: Authorities tell people in Brussels to stay where they are, bringing the city to a standstill.
In Paris, France's top security official said the country was immediately reinforcing security at airports, train stations and metros.
Update 12.55pm: President Michael D Higgins has told Belgium's King Philippe of the sympathy of the Irish people "at this most difficult time".
"I am deeply saddened to learn of the attacks and the tragic loss of lives in Brussels today," he said. "These attacks strike at the fundamental right of all to live in peace.
"These actions must not undermine the will of all Europeans to live and work together."
President Higgins expressed his "sincerest condolences" to the families of all those bereaved and affected by the bombings.
"All of our thoughts are with the people of Brussels at this time of tragedy," he added.
Update 12.05pm: Minister for European Affairs Dara Murphy TD has expressed his horror at news of this morning’s explosions in Brussels.
“I was deeply saddened and horrified to hear the terrible news coming from Brussels this morning of multiple explosions and reported loss of life in Europe’s capital.
“My thoughts are with those affected, the people of Brussels, and with the large Irish community in Brussels and Belgium.
"The Consular Section of the Department of Foreign Affairs and Trade, in conjunction with the Irish Embassy in Belgium, are working with the local authorities.
"Anyone with concerns for family and friends can contact the Consular Division of the Department on +353 1 418 0200.
“Irish citizens in Brussels or Belgium are being advised to exercise caution and closely follow the instructions of local authorities.
"The Department of Foreign Affairs will be updating its guidance in consultation with the Authorities as this situation unfolds."
Update 11.45am: Anthony Deloos, an airport worker for Swissport, which handles check-in and baggage services at the airport, said the first explosion took place near the Swissport counters where customers pay for overweight baggage. He and a colleague said second blast hit near the Starbucks cafe.
"We heard a big explosion. It's like when you're in a party and suddenly your hearing goes out, from like a big noise," Mr Deloos said, adding that shredded paper floated through the air as a colleague told him to run.
"I jumped into a luggage chute to be safe," he said.
Tom De Doncker, 21, check-in agent intern, was near the site of the second explosion.
"I saw a soldier pulling away a body," he said. "It felt like I was hit too" from the concussion of the blast.
Update 11.25am: Ryanair have issued a statement saying: "Brussels Zaventem Airport is closed until 6am tomorrow and as a result, all of our remaining Brussels Zaventem flights have been cancelled.
"Flights to/from Brussels Charleroi are running, although with some delays at the airport. Customers due to travel to/from Brussels Zaventem today should check the Ryanair.com website for the latest information. Our thoughts and prayers are with the families and friends of the victims."
Update 10.44am: Associated Press has quoted Brussels police spokesman Christian De Coninck saying there were deaths at Maelbeek.
"There are victims, serious injury, people have died. I have no idea yet on the numbers of injured or dead," he is quoted as saying.
Update 10.35am: Anthony Barrett, 50, who works for the Wales Audit Office was in his room in the Sheraton Brussels Airport Hotel when the blast occured.
"I looked at the window and could see people fleeing the terminal building," he said.
"Police began evacuating the airport and there were multiple casualties. At one point, I counted 22 people being stretchered into ambulances."
"I could see armed police were taking cover behind a number of parked cars. I don't know if they had a suspect cornered."
Update 10.12am: The Irish Department of Foreign Affairs has increased its security setting for Belgium.
"Any Irish citizens in Belgium should exercise extreme caution and closely follow the instructions of the local authorities," it stated this morning.
Update 10.05am: Ryanair have released a list of cancelled flights due the Brussels attacks, you can view them here.
You can also view a list of diverted Ryanair flights here.
Update 10am: Aer Lingus is offering free changes and refunds to anyone travelling to Brussels today.
#Brussels flights cancelled for Tuesday 22 Mar
EI 631 BRU-DUB
EI 638 DUB-BRU
EI 639 BRU-DUB
Free change/refund online or via +353 1 886 8989— Aer Lingus (@AerLingus) March 22, 2016
Update 9.32am: Zach Mouzoun, who arrived on a flight from Geneva about 10 minutes before the first blast, told BFM television that the second, louder explosion brought down ceilings and ruptured pipes, mixing water with blood from victims.
"It was atrocious. The ceilings collapsed," he said.
"There was blood everywhere, injured people, bags everywhere."
"We were walking in the debris. It was a war scene."
Update 9.22am: Near the entrance to the Maelbeek subway station, not far from the headquarters of the European Union, rescue workers set up a makeshift treatment centre in a local pub.
Brussels blasts: Another explosion target a metro station near EU headquarters https://t.co/ycSZeaNmgO— FRANCE 24 English (@France24_en) March 22, 2016
Dazed and shocked morning travellers streamed from the metro entrances as police tried to set up a security cordon.
"The metro was leaving Maelbeek station when there was a really loud explosion," said Alexandre Brans, 32, wiping blood from his face.
"It was panic everywhere. There were a lot of people in the metro."
First responders ran through the street outside with two people on stretchers, their clothes badly torn.
Update 9.11am: Belgian broadcaster VRT is reporting that at least 13 people have been killed and 35 severely injured in the airport blasts.
#Belgium federal prosecutor confirms 13 dead to Belgian radio.— Alexander Marquardt (@MarquardtA) March 22, 2016
Update 9am: An Associated Press reporter has said that several people were injured in explosion at Maalbeek metro station.
Update 8.24am: It is now being reported that an explosion has been heard at a Brussels metro station close to EU buildings.
Just arrived at Schuman station after walking on the tracks. pic.twitter.com/4xc0YCQmIv— Evan Lamos (@evanlamos) March 22, 2016
Update 8.07am: Belgian authorities have confirmed that at least one person has died in today's explosions at Brussels Airport, the BBC has reported.
Update 7.56am: The BBC has reported that the Belgian fire service has told local media that there are several dead and wounded.
Update 7.40am: Sky News Middle East correspondent Alex Rossi said people were "dazed and shocked".
"The word is definitely two explosions.
Passengers that are still located in other area's in the airport are asked to remain calm and wait for further information.— Brussels Airport (@BrusselsAirport) March 22, 2016
"The thinking here by everybody is that it is some kind of terrorist attack although that hasn't been verified by anyone here at the airport.
There have been 2 explosions at the airport. Building is being evacuated. Don't come to the airport area.— Brussels Airport (@BrusselsAirport) March 22, 2016
"No word too of casualties. Don't know how the explosion took place, the method if you like. But it certainly seems Brussels airport has been targeted in a terrorist attack.
"We are all being moved out of the airport now towards the emergency exit.
"There is a great deal of confusion here. Certainly there are a number of very upset, as you might imagine, very frightened people."
He added: "There are fears that there might be other attackers."
Authorities have released the following emergency number for anyone seeking information about relatives in the area: (+32) 02 506 47 11.— FRANCE 24 English (@France24_en) March 22, 2016
Update 7.35am: The incident came as the Belgian capital was on a state of high alert following the arrest of Paris terror attack suspect Salah Abdeslam in the city last week.
Sky News Middle East correspondent Alex Rossi, who was at the airport en route for Tel Aviv, told the channel: "I could feel the buildings move."
According to reports the incident centred on an American Airlines desk in a departure hall.
Belgian media were reporting several casualties.