Three journalists from Turkey's state-run Anadolu Agency were freed on Sunday after being kidnapped by members of the Kurdistan Workers Party (PKK) while on assignment in the mainly Kurdish southeast, the news agency said.

The journalists were abducted while in the southeastern province of Mardin and held for more than 48 hours, Anadolu said on its website.

The three - a correspondent, a photojournalist and a cameraman - were assigned last week to Mardin's Nusaybin district to cover stories in the region, Anadolu said. Southeast Turkey has been scorched by waves of violence since the July collapse of a ceasefire between the PKK and the Turkish state.

Security sources in the southeast told Reuters the three were believed to have been kidnapped after filming in a PKK-stronghold without permission from the militant group.

The PKK, which has waged a three-decade insurgency for autonomy against the government in which more than 40,000 have been killed, is seen as a terrorist organisation by Ankara, Washington and the European Union.

The government says the PKK, working together with Syrian Kurdish militants, was behind a car bomb in the capital Ankara on Wednesday that killed 28 people in the administrative heart of the city.

A splinter Kurdish militant group, the Kurdistan Freedom Hawks (TAK), has since claimed responsibility for the bombing.

This has been dismissed by the government, which says TAK is shielding the international reputation of the Syrian Kurdish fighters who Washington is backing in the fight against Islamic State group in Syria.