US officials fear that any Turkish raid on the separatist Kurdistan Workers' Party (PKK) -- branded a terrorist group in Ankara and Washington -- would upset the relatively calm northern area of war-torn Iraq.

US President George W. Bush and Turkish Prime Minister Recep Tayyip Erdogan discussed the issue by telephone Saturday for the second time in three days, White House spokeswoman Dana Perino said.

Bush and Erdogan "discussed the continuing PKK terrorist attacks against Turkey. The president told the prime minister that the United States will work with Turkey to deal with this terrorist threat," said Perino.

Bush had stressed "the need to work jointly to address that terrorist threat" when the two leaders broached the same subject in a telephone conversation on Thursday, according to the White House.

The exchange, during which they also discussed Israel's campaign against Hezbollah militants in Lebanon, followed a series of security meetings in Ankara after PKK members killed 15 Turkish security force members.

"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 said 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 told reporters.

Early last week, the United States warned Turkey that a cross-border operation against Kurdish rebel bases in Iraq would be "unwise" after Ankara threatened one if Washington and Baghdad fail to crack down on the PKK.

That drew angry charges of a US double-standard -- backing Israel's operations in Lebanon and Gaza but holding Turkey back -- from Erdogan, who hinted that contingency plans were already being drawn up.

"Terrorism is terrorism everywhere," Erdogan said in Istanbul on July 18. "It is not possible to agree with a mentality that tolerates country A and displays a different attitude when it comes to country B."

"At the end of the day, we know how to take care of our problems," he said. "The competent authorities are working accordingly... We keep ourselves ready against possible developments."

Washington 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 Kurdish-populated region.

The United States' failure to crack down on the PKK has often poisoned its ties with Turkey, a key ally in the Middle East, and has been blamed as a prime reason for growing anti-US sentiment among Turks.

At least 87 PKK rebels and 51 members of the security forces have died this year in southeast Turkey, according to an AFP count.

Kurdish militants also claimed responsibility for 11 bomb attacks in urban centers, in which nine people were killed and nearly 140 injured.