US surveillance planes have been overflying PKK camps in the Qandil mountains near the Iranian border for the past four days, focusing on locations where PKK commanders are based, the Firat news agency quoted sources close to the PKK as saying.

"It is believed that an air strike is imminent," said the agency, considered to be a PKK mouthpiece. "It is reported that the PKK is increasing its counter-measures and will respond strongly in case of an attack."

"The PKK has written to the US authorities in Iraq and asked them to stop the overflights and avoid any steps that could lead to instability," the agency reported on its web site.

None of the Firat report could be independently confirmed.

After talks with Turkish Prime Minister Recep Tayyip Erdogan on Monday, President George W. Bush pledged to provide Ankara with "real-time" intelligence on rebel movements, calling the PKK a "common enemy."

Analysts say a large-scale Turkish incursion into northern Iraq is unlikely, but see Bush's promises of intelligence support as tacit US approval for limited Turkish strikes on PKK targets.

Ankara said Tuesday that it retains the military option in northern Iraq, where the PKK has long taken refuge.

Firat reported that the Iraqi Kurdish authorities, who run a virtually autonomous administration in northern Iraq, have stepped up measures restricting rebel movements, setting up checkpoints on roads leading to PKK camps.

"Only villagers who live in those areas are allowed to travel freely," it said.

Iraq said at the weekend that the government and the Kurdish authorities in northern Iraq were enacting measures to curb the PKK, including new checkpoints.

Firat said the Iraqi Kurds are expected to condone a possible Turkish strike, "showing once again that (they) are part of this plan" against the PKK.

Turkey has accused the Iraqi Kurds of tolerating and aiding the PKK, but they have come under US pressure to act against the rebels to head off a large-scale Turkish incursion.

Tensions along the Turkish-Iraqi border increased after October 21, when PKK militants ambushed a military unit, killing 12 soldiers and capturing eight. The captives were released Sunday.

The PKK, listed as a terrorist group by Ankara and much of the international community, has waged a 23-year armed campign for Kurdish self-rule in Turkey's southeast. The conflict has claimed more than 37,000 lives.