Emergency workers work at the explosion site in Ankara, Turkey March 13, 2016. [Photo/Agencies]

State broadcaster TRT said the car had exploded at a major transport hub, hitting a bus carrying some 20 people near the central Guven Park and Kizilay Square. It said the area was crowded when the explosion happened at 6:43 pm (1643 GMT).

NATO member Turkey faces multiple security threats. As part of a US-led coalition, it is fighting Islamic State in neighbouring Syria and Iraq. It is also battling PKK militants in its southeast, where a 2 1/2-year ceasefire collapsed last July, triggering the worst violence since the 1990s.

The US embassy issued a warning on March 11 that there was information regarding a potential attack on government buildings in the Bahcelievler area of Ankara, several kilometres away from the site of Sunday's blast.

Islamic State militants have carried out at least four bomb attacks on Turkey since June 2015, including a suicide bombing which killed 10 German tourists in the historic heart of Istanbul in January. Local jihadist groups and leftist radicals have also staged attacks in the NATO member country in the past.