The two leaders spoke on the phone Thursday evening, after Ankara threatened a cross-border operation if Washington and Baghdad fail to crack down on bases of the separatist Kurdistan Workers' Party (PKK) in northern Iraq.

The warning followed the killing last week of 15 security force members by PKK militants.

"I told him (Bush) we want to cooperate with the Iraqi government and he said he agrees we should work together on this issue," Erdogan told reporters late Thursday during a visit to northern Cyprus, the Anatolia news agency reported.

"But I also told him that the limits of our tolerance have been seriously breached and we cannot just put aside the fact that we had 15 martyrs in three days," Erdogan said.

The army stepped up action against the PKK in Turkey's mainly Kurdish eastern and southeastern regions, killing four militants in a clash in the countryside in Van province Friday, security officials said.

Four soldiers were injured, two of them seriously.

PKK militants, meanwhile, opened fire on a military helicopter flying over the rugged mountains of Sirnak province, which borders Iraq and Iran, killing one soldier, officials said.

The soldier, hit by a bullet that pierced the helicopter's window, was among a group of discharged troops who were leaving the area.

Ankara charges that Kurdish-run northern Iraq has become a springboard for PKK attacks inside Turkey, where the rebels enjoy unrestricted movement and are able to easily obtain weapons and explosives.

Like Ankara, Washington considers the PKK a terror organization and has pledged support to its NATO ally Turkey in combating the group.

But it has been reluctant to crack down on the PKK in northern Iraq, arguing that allied forces are overwhelmed by violence in other parts of the country and that military action in the north could destabilize the relatively calm region.

Washington has warned Turkey against unilateral cross-border action, drawing accusations from Erdogan that it is using double standards -- a reference to US support for Israeli offensives against Islamist militants in Lebanon and the Gaza Strip.

Thousands of PKK rebels have moved to northern Iraq since 1999, when the group declared a unilateral ceasefire after the capture of its leader Abdullah Ocalan, now serving a life sentence for treason.

The truce was called off in June 2004 and PKK attacks on government targets have markedly escalated since.

At least 91 PKK rebels and 53 members of the security forces have been killed this year, according to an AFP count.

Kurdish militants have also claimed responsibility for 11 bomb attacks in urban centres across Turkey this year, in which nine people were killed and nearly 140 injured.

The Kurdish conflict has claimed more than 37,000 lives since 1984 when the PKK took up arms for self-rule in the southeast.