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.

Deputy Prime Minister and government spokesman Numan Kurtulmus said 28 people including soldiers and civilians had been killed and 61 wounded in the blast, which took place near a busy intersection less than 500 metres from parliament during the evening rush hour.

"We will continue our fight against the pawns that carry out such attacks, which know no moral or humanitarian bounds, and  the forces behind them with more determination every day," President Tayyip Erdogan said in a statement on Wednesday. 

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, cancelled the trip, an official in his office said. Erdogan postponed a planned visit to Azerbaijan.

White House, German chancellor condemn attack

"I'm telling the Turkish people: we as Germans are sharing your pain," German Chancellor Angela Merkel said in a statement Wednesday. "In the battle against those responsible for these inhuman acts we are on the side of Turkey."

The White House also condemned the attack Wednesday night. 

"We stand together with Turkey, a NATO ally, a strong partner, and a valued member of the counter-ISIL coalition in the face of this attack," said Ned Price, a spokesman for the White House National Security Council, using an alternate acronym for the Islamic State group. 

U.N. Secretary-General Ban Ki-moon also condemned the explosion and "hopes the perpetrators of this terrorist attack will be swiftly brought to justice," U.N. spokesman Stephane Dujarric said.

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A Turkish soldier stands guard near the site of the explosion. (Umit Bektas/Reuters)

The attack, the latest in a series of bombings mostly blamed on Islamic State over the past year, comes as the NATO member gets dragged ever deeper into the war in neighbouring Syria and tries to contain some of the fiercest violence in decades in its restive and largely Kurdish southeast.

A senior security source said initial signs indicated that Kurdish militants from the outlawed Kurdistan Workers Party (PKK) were responsible. Separate security sources in the 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 witness told Reuters. "We could immediately hear ambulance and police car sirens rushing to the scene."

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Turkish police officers close a street after an explosion in Ankara, Turkey on Feb. 17, 2016. (Umit Bektas/Reuters)

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 pm (1630 GMT), sent a large plume of smoke above central Ankara.

Multiple security threats

Turkey, a NATO member, faces multiple security threats. It is part of a U.S.-led coalition fighting the Islamic State in Iraq and Syria (ISIS), 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 labour 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. 

with files from The Associated Press