An ambulance arrives to the site of an explosion in Ankara, Turkey February 17, 2016. [Photo/Agencies]

Prime Minister Ahmet Davutoglu, who had been due to attend meetings in Brussels on the migration crisis on Thursday, cancelled the trip, an official in his office said. Erdogan postponed a planned visit to Azerbaijan.

There was no immediate claim of responsibility for the bombing.

"I heard a huge explosion. There was smoke and a really strong smell even though we were blocks away," a witness said. "We could immediately hear ambulance and police car sirens rushing to the scene."