DIYARBAKIR, Turkey (Reuters) - A roadside bomb ripped through an armoured vehicle and killed three policemen in southeast Turkey on Tuesday (Dec 15), hours after Ankara pledged to prevent Kurdish militants from "spreading the fire" from Syria and Iraq into the country.

Five police officers were wounded in the blast, detonated by remote control on a country road between the main southeastern city of Diyarbakir and the town of Silvan, security officials at the scene of the mangled wreckage told Reuters.

Since the collapse of a ceasefire with the militant Kurdistan Workers Party (PKK) in July, the mainly Kurdish southeast has been wracked by clashes between security forces and PKK fighters and subjected to frequent curfews.

On Monday, two men were shot dead in Diyarbakir, while security forces have killed six militants in the province of Mardin since Friday. Further clashes erupted in Diyarbakir on Tuesday, wounding a police officer, security sources said.

Prime Minister Ahmet Davutoglu said security forces would crack down on the PKK militants to frustrate efforts to "spread the fire" from neighbouring Iraq and Syria to Turkey.

"Security operations are being conducted," he said. "We will turn all districts including Cizre and Silopi within this ring of fire into a place of peace, stability and freedom," he told a news conference in Ankara.

Clashes broke out in the town of Cizre near the Syrian border on Tuesday afternoon as security forces entered one district and came under fire from PKK militants but there were no immediate reports of casualties there, security sources said.

Witnesses said police deployed armoured vehicles on hills overlooking both Cizre and Silopi after both were placed under curfew overnight.

"CLEANSING"

In the town of Nusaybin on the Syrian border, gunfire and explosions rang out overnight after a curfew was imposed, with one blast cutting power to some areas, while police fired tear gas and water cannon at protesters, witnesses said.

Davutoglu said the army had established control along the mountainous Iraqi border after "cleansing the mountains of terrorists", and that the PKK had since focused on turning people against the state in urban areas.

"We will not make concessions to terror and will not tolerate in any way these structures which are pawns of various foreign forces who want to drag Turkey into a dark future."

Abdulkadir Selvi, a columnist with the pro-government Yeni Safak newspaper, sensed in Davutoglu's recent comments that preparations were under way to step up anti-PKK operations.

"This time, the emphasis in operations will be more on soldiers," he said. "The prime minister has spoken quite decisively, saying 'all districts will be cleansed of terrorist elements, from street-to-street, house-to-house if necessary'."

According to data compiled by the Human Rights Foundation of Turkey, 52 curfews have been imposed since mid-August across seven Turkish provinces in the southeast, affecting areas where some 1.3 million people live.

The PKK launched its insurgency in 1984 and more than 40,000 people have been killed in the conflict. Peace talks between its jailed leader Abdullah Ocalan and the state ground to a halt early this year. The PKK is designated as a terrorist group by Turkey, the United States and European Union.