In a telephone call Monday with his Turkish counterpart Abdullah Gul, US President George W. Bush had offered support for Turkish efforts to counter attacks by the outlawed Kurdistan Workers' Party (PKK).

Bush "reaffirmed our commitment to work with Turkey and Iraq to combat PKK terrorists operating out of northern Iraq," White House national security council spokesman Gordon Johndroe said.

While Johndroe did not elaborate on the nature of the cooperation, the Chicago Tribune reported Tuesday that Bush had told Gul that US officials were seriously looking into options beyond diplomacy.

US officials have considered launching cruise missile against PKK targets, but air strikes using manned aircraft was an easier option, an unnamed US official told the Tribune.

"In the past, there has been reluctance to engage in direct US military action against the PKK," the official told the Tribune.

"But the red line was always, if the Turks were going to come over the border, it could be so destabilizing that it might be less risky for us to do something ourselves.

"Now the Turks are at the end of their rope, and our risk calculus is changing," the official said.

US officials fear a unilateral cross-border Turkish incursion could further destabilise Iraq.

The talk of a joint operation was confirmed by Turkish Prime Minister Recep Tayyip Erdogan as he flew to Britain on Monday.

"We may conduct a joint operation with the United States against the PKK in northern Iraq ... We expect to work jointly, just as we do in Afghanistan," Erdogan told reporters on the plane.

Erdogan said he had received a signal that Washington might become involved during a telephone conversation with US Secretary of State Condoleezza Rice on Sunday.

"She was worried. I saw she was in favour of a joint operation," Erdogan said. "She asked for a few days' time and said she would come back to us."

Erdogan has threatened military action in northern Iraq unless Baghdad clamps down on the PKK bases there and turns over the rebel leaders that Ankara accuses of masterminding cross-border attacks on its military.

In a Monday video-conference with Iraqi Prime Minister Nuri al-Maliki, Bush pressed for more action from Baghdad and Iraq's Kurdish north against the PKK.

Maliki agreed with Bush "that Turkey should have no doubt about our mutual commitment to end all terrorist activity from Iraqi soil," Johndroe said.

"They agreed to work together, in cooperation with the Turkish government, to prevent the PKK from using any part of Iraqi territory to plan or carry out terrorist attacks."

The crisis over the PKK has coincided with a downturn in US-Turkish ties caused by a push in the opposition-controlled US Congress to describe the World War I massacre of Armenians by Ottoman Turks as "genocide".

After the House Foreign Affairs Committee approved the resolution, Turkey recalled its ambassador to Washington and hinted it would prevent the US military from using the Incirlik air base in southern Turkey as a staging post for supplies heading to US operations in Iraq and Afghanistan.

Incirlik is a vital staging post for regional US military operations.