"Nothing is impossible," said Matthew Bryza, deputy assistant secretary of state for European and Eurasian affairs.

"If we cooperate together and deepen this cooperation why should anything be impossible?"

His remarks were a response to recent comments by Iraqi President Jalal Talabani, also a Kurd, that the Iraqi authorities were unable to arrest commanders of the rebel Kurdistan Workers' Party (PKK), because they were holed up in remote mountainous areas.

Turkey, where the PKK has been waging a 23-year insurgency, has threatened to launch a military incursion into northern Iraq unless Baghdad and Washington crack down on the separatist bases there.

"Anybody anywhere in the world sees how seriously Turkey is taking this issue," said Bryza, who acknowledged that the United States also needed to make good on its pledges to help eliminate the PKK threat.

"We'll deliver on those promises. We are working on it... with the Turkish government and the Iraqi government," Bryza said. "We know we need to produce concrete results".

Bryza also said that Washington was pushing hard for the release of eight Turkish troops captured by the PKK during a weekend ambush near the Iraqi border.

"We are doing what we can, working with the Turkish government and the Iraqi government to make sure that the remaining hostages are freed," he said.

In a statement posted Thursday on the website of a pro-Kurdish news agency, the PKK said the eight soldiers were being "held in guerrilla areas in northern Kurdistan" -- rebel parlance for Kurdish-majority southeast Turkey.

Bryza insisted that Washington wants to help in meeting Turkish demands for the arrest of PKK leaders in northern Iraq, cutting the group's logistical supplies and closing down its political front offices there.

"We've improved and increased our intelligence sharing cooperation with Turkey... This is an ongoing effort," he told reporters.