Twenty-eight people were killed and dozens wounded in Turkey's capital Ankara on Wednesday when a car laden with explosives detonated next to military buses near the armed forces' headquarters, parliament and other government buildings.
The Turkish military condemned what it described as a terrorist attack on the buses as they waited at traffic lights in the administrative heart of the city.
A government spokesman said 28 people had been killed and 61 wounded in the blast, which took place near a busy intersection less than 500 meters from parliament during the evening rush hour.
Justice Minister Bekir Bozdag described the attack as an act of terrorism and told parliament, which was in session when the blast occurred, that the car had exploded on a part of the street lined on both sides by military vehicles.
Prime Minister Ahmet Davutoglu, who had been due to leave for meetings in Brussels later on Wednesday, canceled the trip, an official in his office said. President Tayyip Erdogan postponed a planned visit to Azerbaijan.
A senior Turkish security source said initial signs indicated that Kurdish militants from the outlawed Kurdistan Workers Party (PKK) were responsible. Separate security sources in the mainly Kurdish southeast, however, said they believed Islamic State militants may have been behind the bombing.
There was no immediate claim of responsibility.
"I heard a huge explosion. There was smoke and a really strong smell even though we were blocks away," a Reuters witness said. "We could immediately hear ambulance and police car sirens rushing to the scene."
A health ministry official said the authorities were still trying to determine the number of dead and wounded, who had been taken to several hospitals in the area.
Images on social media showed the charred wreckage of at least two buses and a car. The explosion, which came shortly after 6:30 p.m local time, sent a large plume of smoke above central Ankara.
Turkey, a NATO member, faces multiple security threats. It is part of a US-led coalition fighting Islamic State in neighboring Syria and Iraq, and has been shelling Kurdish militia fighters in northern Syria in recent days.
It has also been battling PKK militants in its own southeast where a 2-1/2 year ceasefire collapsed last July, plunging the region into its worst violence since the 1990s.
The PKK, which has fought a three-decade insurgency for Kurdish autonomy, has frequently attacked military targets in the past, although it has largely focused its campaign on the mainly Kurdish southeast.
Wednesday's bombing comes after an attack in Ankara in October blamed on Islamic State, when two suicide bombers struck a rally of pro-Kurdish and labor activists outside the capital's main train station, killing more than 100 people.
A suicide bombing in the historic heart of Istanbul in January, also blamed on Islamic State, killed 10 German tourists.