ANKARA - A car bomb ripped through a busy square in central Ankara Sunday, killing at least 28 people and wounding 75, officials said, the latest in a spate of attacks to hit Turkey.

Ambulances rushed to the scene on Kizilay square, a key commercial and transport hub close to the city's embassy area, where the blast reduced several vehicles including a bus to burnt-out wrecks. The attack comes just weeks after the city was hit by a suicide car bombing on February 17 targeting the military that killed 29 people, claimed by a dissident faction of the oulawed Kurdistan Workers' Party (PKK).

The provincial governor's office said there were 27 dead and 75 wounded in Sunday's attack. "The blast was caused by a vehicle packed with explosives close to Kizilay square," an official statement said.

Medical sources told AFP the wounded had been taken to 10 different hospitals around the city, with a dozen said to be in a very serious condition.

Turkey has been hit by a spate of deadly attacks since the middle of last year, most of them blamed on the Islamic State (IS) group, including a double suicide bombing in Ankara in October that left 103 people dead.

Coming so soon after the February bombing, Sunday's attack will raise fresh questions about Turkey's ability to manage the twin security threat posed by IS and Kurdish rebels, as Ankara presses the European Union to speed up its membership process in return for help with the migrant crisis.

The Kurdistan Freedom Falcons (TAK), linked to the PKK, said it carried out the February bombing in Ankara as revenge for operations by the Turkish military in the southeast of the country and warned foreign tourists not to visit the country.

A two-year ceasefire between the government and Kurdish rebels collapsed in the middle of last year and since December security forces have been waging a major campaign against the PKK in the southeast of the country.

Strict 24-hour curfews were imposed in a number of Kurdish-dominated towns and cities to allow the military and police to pursue the fight against fighters who had dug trenches and put up barricades.

Sunday's attack came hours before curfews were due to take effect in two more towns in the southeast as a prelude to fresh military operations.

Authorities said restrictions would be slapped on Yuksekova, near the Iranian border, and Nusaybin, on the frontier with Syria, to "restore order and security" following an increase in "terrorist activity".

Ankara has vowed to wipe out the PKK, classed as a terrorist organisation by Turkey and its Western allies, and has said "clean-up" operations in Yuksekova, Nusaybin and Sirnak, a third Kurdish city, are imminent.

Meanwhile, Turkey on Sunday slapped a curfew on two border towns in the Kurdish-dominated southeast ahead of a looming military "clean-up" operation as it eased a lockdown in Diyarbakir.

Turkish troops have been waging a major - and controversial - offensive against the outlawed Kurdistan Workers' Party (PKK) since December, imposing strict 24-hour curfews in a number of towns and cities in the southeast.

Ankara has repeatedly imposed curfews for military operations in southeastern urban centres, and on Sunday said restrictions would be slapped on two more towns - Yuksekova, near the Iranian border, and Nusaybin, on the frontier with Syria.

The aim was to "restore order and security" following an increase in "terrorist activity", local authorities said.

Several hours before the curfew was to start, an explosion rocked Yuksekova, injuring four people, Dogan news agency reported.

The cause of the blast was not immediately clear.

Ankara has vowed to wipe out the PKK, classed as a terrorist organisation by Turkey and its Western allies, and has said "clean-up" operations in Yuksekova, Nusaybin and Sirnak, a third Kurdish city, are imminent.

As the new restrictions were announced, Turkey eased the curfew in part of Diyarbakir, the biggest Kurdish majority city, which has been under lockdown since December.

From 8:00 am (0600 GMT) on Sunday, residents of part of the city's historic Sur district were allowed back onto the streets, where some buildings have been badly damaged in the operation, an AFP correspondent said.

Residents forced from the area by the clashes and the curfews returned to inspect their damaged homes, carrying belongings in suitcases and pushcarts.

Other parts of Sur, a UNESCO world heritage site, remain under curfew to allow the authorities to "capture terrorists" and "clear explosives and booby-traps", according to the local governor.

Seven PKK fighters were killed in clashes with security forces in Sur on Sunday, local media reported, citing police sources.

The police and army launched an operation in the narrow streets of Sur in early December aimed at retaking control of areas seized by armed PKK activists, who dug trenches and put up barricades.

Critics say the clashes have caused major damage and forced nearly 50,000 people in Sur from their homes since the start of December. Up to 70,000 people were living in the area before the violence erupted.

Army high command said this week its operation in Sur had killed 279 members of "the separatist terrorist organisation", the PKK, but gave no toll for security forces. Local media have put the figure in the dozens.

The pro-Kurdish Peoples' Democratic Party (HDP) says dozens of civilians have also been killed.

Violence flared last summer between Kurdish rebels and government forces after a deadly bombing in a Kurdish majority town, shattering a 2013 ceasefire reached after secret talks between PKK leader Abdullah Ocalan and Ankara.

Over 40,000 people have been killed since the PKK took up arms in 1984 demanding an independent state for Kurds. Since then the group has narrowed its demands to greater autonomy and cultural rights.