Update 5.27pm: At least five people have been killed and ten others injured in an explosion that rocked the Turkish capital, the governor of Ankara has said.
BREAKING: Ankara governor raises death toll in explosion to 18, says at least 45 others wounded— The Associated Press (@AP) February 17, 2016
Mehmet Kilicer said officials believe the explosion was caused by a car bomb.
The explosion occurred during rush hour in an area some 300 metres away from military headquarters.
"According to preliminary assessments, five people died and 10 people were injured," Mr Kiliclar told the state-run Anadolu Agency. "It is believed that a bomb-laden car caused the explosion."
News reports said some cars caught fire and dozens of ambulances were sent to the scene. Dark smoke could be seen billowing from a distance.
Police told the Associated Press they are investigating the cause of the explosion.
It was not clear who was behind the bombing. Kurdish rebels, the Islamic State (IS) group and a leftist extremist group have carried out attacks in the country recently.
The attack comes at a tense time when the Turkish government is facing an array of challenges.
A fragile peace process with the Kurdish rebels collapsed in the summer.
Security forces have been engaged in large-scale operations against Kurdish militants in the south-east since December, imposing controversial curfews in flashpoint areas, and the fighting has displaced tens of thousands of civilians.
Turkey has also been helping efforts led by the US to combat IS in neighbouring Syria and has faced several deadly bombings in the last year that were blamed on IS.
The Syrian war, meanwhile, is raging along Turkey's southern border.
Recent airstrikes by Russian and Syrian forces have prompted tens of thousands of Syrian refugees to flee to Turkey's border.
Turkey, so far, has refused to let them in, despite being urged to do so by the United Nations and European nations, but is sending aid to Syrian refugee camps right across the border.
Turkey's prime minister has cancelled a planned visit to Brussels following the explosion in Ankara.
Ahmet Davutoglu had been scheduled to travel to Brussels later on today.
Instead, Mr Davutoglu attended a security meeting with President Recep Tayyip Erdogan and other officials.