Kurdish militants have set off a roadside bomb that killed four Turkish police officers as they travelled in an armoured vehicle in the southeast of the country.

Militants of the Kurdistan Workers' Party (PKK) have mounted daily attacks against security forces since a two-year ceasefire ended in July. Roadside bombs are a common PKK tactic.

The attack occurred as the police officers joined the local fire brigade in travelling to the scene of a fire which PKK militants were believed to have started in a school, according to the governor's office in Mardin province.

Turkish warplanes bombed PKK positions on Wednesday after one soldier was killed in the same region. State media said 20 militants were killed in those air strikes.

More than 70 members of Turkey's security forces have been killed since the wave of PKK attacks began in late July, when the government also launched air strikes on the group's camps in northern Iraq.