Update 1.50pm: Families of the Sri Lankan crew members held captive on an oil tanker are appealing to Somali pirates to release the men unharmed.
The hijacking on Monday was the first such seizure of a large commercial vessel off Somalia since 2012.
It came as a surprise to the global shipping industry as international patrols had suppressed pirate hijackings for several years.
The European Union anti-piracy operation in the region announced late on Tuesday that the armed men are holding the crew captive and demanding a ransom for the ship's release.
Namali Makalandawa, the sister of the tanker's chief officer Premnath Ruwan Sampath, said on Wednesday that families have tried to contact the shipping company's office in Dubai but their calls were not answered.
Ms Makalandawa said they fear for their relatives' lives.
Earlier: Armed men are demanding a ransom for the release of an oil tanker seized off the coast of Somalia and the crew is being held captive, the EU anti-piracy operation in the region said.
An EU Naval Force statement said the operation had finally made contact with the ship's master, who confirmed that armed men were aboard the Comoros-flagged tanker Aris 13.
Monday's hijacking was the first such seizure of a large commercial vessel off Somalia since 2012.
It came as a surprise to the global shipping industry as patrols by the navies of Nato countries, as well as China, India and Iran, had suppressed Somali pirate hijackings for several years.
However, the United Nations warned in October that the situation was fragile and that Somali pirates "possess the intent and capability to resume attacks".
One expert said some in the region had let down their guard as the situation calmed.
Nato ended its anti-piracy mission off Somalia in December.
A Somali pirate who said he was in touch with the armed men aboard the tanker said the amount of ransom to demand had not yet been decided.
Bile Hussein told The Associated Press that the armed men have locked most of the crew in one room and cut off communication lines.
"Their main concern now is a possible rescue attempt, so that's why all communications were cut off in the afternoon," he said.
The Aris 13, manned by eight Sri Lankan sailors, was carrying fuel from Djibouti to Somalia's capital, Mogadishu, when it was approached by men in two skiffs, said John Steed, the director of Oceans Beyond Piracy.
The EU statement said the ship's master issued a mayday alert.
Forces have been sent to free a hijacked ship, "We are determined to rescue the ship & its crew. Our forces set off to Alula. officials says pic.twitter.com/D3YSzXV6Ng— Somalia Live Update (@HassanIstiila) March 15, 2017
An official in Somalia's semi-autonomous state of Puntland said over two dozen men boarded the ship off the country's northern coast, an area known to be used by weapons smugglers and members of the al Qaida-linked extremist group al-Shabab.
The ship was anchored off the town of Alula, said Salad Nur, a local elder.
"The ship is on the coast now and more armed men boarded the ship," he told the AP by phone.
The EU Naval Force said it had passed the information from its contact with the ship's master to the oil tanker's owners and an investigation was under way.
Sri Lanka's foreign ministry said in a statement it was in touch with shipping agents and officials abroad for more information to help ensure the crew's "safety and welfare".
A UN shipping database shows the Aris 13 is owned by Armi Shipping SA, whose address is listed in care of Aurora Ship Management FZE, a company based in the United Arab Emirates.
Calls and emails to Aurora went unanswered.
Australian government records from 2014 list the ship's owner as Flair Shipping Trading FZE in the UAE.
An address listed for Flair Shipping in Dubai was for a company called Flair Oil Trading DMCC.
A woman there told an AP reporter the firm was not connected to the ship and directed him to another office.
When no one answered there, the reporter returned to find Argyrios Karagiannis, the managing director of Flair Shipping, entering the first office.
"We will not be releasing any information," Mr Karagiannis said before shutting the door.
The incident involving the Aris 13 represents the first commercial pirate attack off Somalia since 2012, Mr Steed of Oceans Beyond Piracy said.
"The pirates never went away, they were just doing other forms of crime and if any of the measures reduce (which they have, or ships take risks) the pirates are poised to exploit the weakness," he said in an email.
Somali pirates usually hijack ships and crew for ransom.
They do not normally kill hostages unless they come under attack.
Piracy off Somalia's coast was once a serious threat to the global shipping industry.
It has lessened in recent years after an international effort to patrol near the country, whose weak central government has been trying to assert itself after a quarter-century of conflict.
In that time, concerns about piracy off Africa's coast have largely shifted to the Gulf of Guinea.
But frustrations have been rising among local fishermen, including former pirates, at what they say are foreign fishermen illegally fishing in local waters.
Nur, the local elder, told AP that young fishermen including former pirates have hijacked the ship.
"They have been sailing through the ocean in search for a foreign ship to hijack since yesterday morning and found this ship and boarded it," he said on Tuesday.
"Foreign fishermen destroyed their livelihoods and deprived them of proper fishing."