ANKARA - 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 metres 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, cancelled 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 pm (1630 GMT), sent a large plume of smoke above central Ankara. Turkey, a NATO member, faces multiple security threats. It is part of a U.S.-led coalition fighting Islamic State in neighbouring 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 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.

Moreover, Turkey said on Wednesday it was seeking to create a "safe line" inside Syria that would include the flashpoint northern town of Azaz near the Turkish border.

"We want to form a 10-kilometre (six-mile) safe line inside Syria, including Azaz," Deputy Prime Minister Yalcin Akdogan told A Haber television in an interview. Turkey has long pressed for a safe zone, backed up by a no-fly zone, to protect its borders and provide protection for refugees on Syrian soil.

Akdogan did not elaborate on the dimensions of the proposed zone or how it could be created. Alarmed by the advances of Syrian Kurdish forces in Aleppo province near the border, Ankara has in recent days bombed their positions, defying international calls for a halt to the strikes.

German Chancellor Angela Merkel on Monday backed calls for a no-fly zone in Syria, saying it "would be helpful if there could be such an area, where none of the parties are allowed to launch aerial attacks." Moscow, which has carried out airstrikes to help President Bashar al-Assad regime's ground offensive in northern Syria, said Wednesday that any decision would have to be approved by Damascus. "Any decisions on creating some sort of no-fly zones cannot be taken without the agreement by the receiving side and without a relevant UN Security Council decision," deputy foreign minister Gennady Gatilov told Interfax.

Turkey accuses Kurdish fighters from the Democratic Union Party (PYD) in Syria of links to the banned Kurdistan Workers' Party (PKK), which has waged a decades-long insurgency against the Turkish state.

The fate of Azaz is of particular concern to Turkey, with Prime Minister Ahmet Davutoglu making clear Ankara will not allow Kurdish fighters to take it from anti-regime rebels. The Turkish shelling in northern Syria has troubled an already complicated situation in Aleppo province, where regime forces have made significant advances with backing from Russian air strikes. Turkey has set up several camps inside Syria near Azaz to house Syrians fleeing the regime's strikes.

In the meantime, President Recep Tayyip Erdogan on Wednesday said there was no question of Turkish forces stopping their bombardment of Syrian Kurdish fighters, vowing not allow the creation of a Kurdish stronghold in northern Syria.

Turkish Firtina howitzers stationed in Turkey's southern region of Kilis shelled positions of Kurdish fighters in Syria for the fifth day in the row, in response to incoming fire, reports said. "They (the West) tell us to stop shelling" the Kurdish fighters in Syria, Erdogan said in a televised speech. "Forgive me, but there is no question of us doing such a thing. Whoever fires shells on Turkey will get not just a tit-for-tat response but an even greater one," he added. "We will not allow a new Qandil on our southern border" with Syria, Erdogan added, referring to the mountain in northern Iraq which for years has been stronghold of Kurdistan Workers Party (PKK) militants.

Western countries including the United States and France have urged Turkey to stop the firing, fearing it will endanger an already beleaguered deal, agreed in Munich last week, for a cessation of hostilities.

After calm in the morning, the Turkish howitzers renewed firing on positions of the Syrian Kurdish Democratic Union Party (PYD) and its People's Protection Units (YPG) militia from 1445 GMT, the Dogan news agency reported.

Erdogan meanwhile stepped up a war of words with the United States, which supports the PYD and YPG as the best fighting force on the ground against Islamic State (IS) jihadists. "The fact that the United States is continuing to support the YPG is something I find hard to understand," said Erdogan. "Aren't we NATO allies?" he said, addressing Washington. "Are you our friend or the friend of the PYD?"

"We know that we need friends. But if you don't see us as friends then please say it loud and clear." Washington recognises the PKK as a terror group but not the PYD and YPG. State Department spokesman Mark Toner said the United States urged Turkey to cease artillery fire across the border. "We just think it escalates tensions in the region."

Turkey fears the Syrian Kurds want to carve out a continuous autonomous region in northern Syria stretching from the Iraqi border almost to the Mediterranean. Ankara is concerned the Kurds will now take a so-called "corridor" east of the town of Azaz on the Turkish border still in control of rebels to link up two Kurdish-held areas.

"We will not hesitate to take the necessary steps against all terror organisations in Syria," said Erdogan. "If someone is trying to test the limits of Turkey's patience then they should know this limit has been reached."

Turkey has long pressed for a safe area backed by a no fly zone in this region extending several kilometres into the border to ensure its security and house Syrian refugees. "We want to form a 10-kilometre (six-mile) safe line inside Syria, including Azaz," Deputy Prime Minister Yalcin Akdogan told A Haber television in an interview.

But the idea has never found much enthusiasm from the United States, a failure which Erdogan said had allowed Russia to seize the military initiative. "Hey America! You did not want the no-fly zone and now the Russian planes are bombing the area and thousands of people are dying," he added.

A senior Turkish official said Tuesday that Turkey was in favour of a ground operation in Syria to end the fighting, but only in cooperation with its Western and Gulf allies. But Erdogan said: "We are not in search of an adventure and we are not seeking to throw someone else into an adventure."