Turkey on Monday held four suspects over a suicide car bombing that killed at least 36 people in Ankara, as warplanes pounded Kurdish rebel bases in northern Iraq over the attack, the capital's third in five months.

There was no immediate claim of responsibility for the latest carnage, which reduced cars and buses to charred husks on a busy road in the heart of the city on Sunday evening, wounding more than 120 people.

But Ankara believes one of the bombers was a woman with ties to the outlawed Kurdistan Workers' Party (PKK), a Turkish official told AFP on Monday.

Turkish police detained four people near the Syrian frontier on Monday, state-run Anatolia news agency reported, acting on a tip-off that the car used in the bombing had been bought in Sanliurfa, a Kurdish-dominated town some 50 kilometres (30 miles) from the border.

Dogan news agency for its part said six people had been arrested.

The fact that extremists were able to strike again in the heart of the capital, so close to many sensitive buildings and so soon after February's attack will raise questions about Turkey's ability to deal with the twin threat of Kurdish rebels and the Islamic State (IS) group.

Hours after the attack, Turkish fighter bombers hit PKK arms depots and shelters in mountainous northern Iraq, the army said, quoted by Anatolia.

Health Minister Mehmet Muezzinoglu on Monday gave a new toll of 37 from Sunday's blast targeting a bus stop, but said this included at least one attacker and possibly two. The first funerals for the victims were held on Monday.

The military said the PKK targets were hit "with precision", with a rebel spokesman confirming the strikes.

- Kurdish link -

Sunday's attack bore similarities to another suicide car bombing on a convoy of military buses which killed 29 people in Ankara on February 17.

That attack was claimed by the Kurdistan Freedom Falcons (TAK), linked to the PKK, as revenge for Turkish military operations in the southeast. The TAK warned of more attacks to come, including on tourist areas.

The PKK, classed as a terrorist group by Ankara and its Western allies, launched a bloody insurgency in 1984 demanding an independent state for Kurds.

Prime Minister Ahmet Davutoglu said Turkey had "concrete information" on the group behind Sunday's attack, saying results from the inquiry would be made public soon.

"One attacker is believed to be a woman with links to the PKK," a Turkish official told AFP. Local media reported her name as Seher Cagla Demir, saying she was identified by fingerprints.

Sunday's attack hit Ankara's Kizilay square, a bustling commercial and transport hub close to the parliament, prime minister's office and foreign embassies.

Turkey has been hit by a string of major attacks since the middle of last year, most of them blamed on IS. Three have targeted Ankara, including a double suicide bombing in October that left 103 people dead.

Just two days before Sunday's bombing the US embassy warned of a possible plot to attack central Ankara, advising American citizens to avoid the area.

President Recep Tayyip Erdogan said late Sunday that the government would "never abandon its right to legitimate defence against the terrorist menace."

- Growing anger -

Erdogan's party won parliamentary elections four months ago campaigning as the only sure bulwark against rising insecurity, but the ongoing bloodshed is causing anger.

"People have been talking about another bomb attack coming for more than a week but the government took no precautions and didn't warn anyone," Nihat Gorgulu, the uncle of one of the victims, told AFP.

"We are very afraid because the government doesn't care about the people of this country."

Kemal Kilicdaroglu, the head of the main opposition Republican People's Party, slammed the government for not appointing a new police chief for Ankara after the old one was sacked following the October attack.

"We do not deserve this intolerable situation where parents are burying their children -- Turkey is not well governed but some people are turning a blind eye," he said.

Ankara has vowed to smash the PKK, and security forces have been waging a major campaign against the rebels since December, following the collapse of a ceasefire in the middle of last year.

"The 'uprising' launched by the PKK has not worked. Even the Kurdish population has distanced itself from its operations in the southeastern towns," Can Acun, an analyst with Turkish thinktank SETA, told AFP.

"In frustration the PKK seems to have chosen to go for more serious acts."

Officials rejected the TAK claim for February's attack, insisting it was the work of the Syrian Kurdish People's Protection Units (YPG), which it regards as a branch of the PKK. Both organisations have denied it.