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

The journalists were abducted and held for more than 48 hours, Anadolu said on its website. The correspondent, photojournalist and cameraman had been assigned last week to Nusaybin district of Mardin province.

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 region has been swept by violence since the collapse in July of a ceasefire between the government and the PKK, which has waged an insurgency for Kurdish autonomy for decades.

The PKK is seen as a terrorist organisation by Ankara, Washington and the European Union.

Three members of the security forces were killed in attacks by militants on Sunday, the military said in a statement. One was killed in Sirnak province and two in the Sur district of Diyarbakir, the southeast's largest city.

Hundreds of protesters clashed with security forces in the city over the round-the-clock curfew that has been in place in Sur since December.

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.

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

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

Twenty-one suspects who have been detained in an investigation over the Ankara bombing appeared in court in the capital on Sunday.