(Adds Iraqi government spokesman, Istanbul dinner)

By Sue Pleming and Paul de Bendern

ISTANBUL, Nov 2 (Reuters) - The United States promised on Friday to try to help stop Kurdish guerrillas operating in northern Iraq in an attempt to stave off a threatened Turkish military intervention that could destabilise the region.

U.S. Secretary of State Condoleezza Rice called the militants a "common enemy". But she did not spell out what Washington might do to stop them using Iraq as a base for attacks on Turkey, where they seek an independent Kurdish state.

"We all need to redouble our efforts and the United States is committed to redoubling our efforts," Rice told a news conference with Turkish Foreign Minister Ali Babacan in Ankara.

Babacan expressed Turkish frustration at the lack of action so far against the outlawed Kurdistan Workers Party (PKK). "This is where the words end and action needs to start," he said.

Rice said the PKK would be discussed at a meeting between herself and ministers from Turkey and Iraq on the sidelines of an Iraq neighbours' conference in Istanbul on Saturday.

Turkey, a NATO member with the alliance's second biggest army, has sent up to 100,000 troops to the Iraqi border, backed by tanks, artillery and aircraft. Baghdad and Washington have urged Ankara to refrain from a major operation in an area that has been spared the worst of the violence in Iraq.

"We know fully well that our neighbour Turkey is not willing to destabilise Iraq. They are willing to support Iraq. Both of us are against the common enemy, the PKK terrorist organisation that threatens Turkey and threatens Iraq as well," Iraqi government spokesman Ali al-Dabbagh told reporters in Istanbul.

Ankara has said it will take cross-border action shortly to root out some 3,000 PKK militants in northern Iraq if U.S. and Iraqi forces fail to follow up on past pledges of action.

In remarks that appeared to justify Turkish scepticism about Iraq's ability to tackle the PKK, al-Dabbagh said: "It is not in our capacity, it is not even in the capacity of Turkey. (Our shared border) is like the Afghan-Pakistan border.

"The United States has bombed al-Qaeda there for 5-6 years and they cannot get rid of them there."

Nearly 40,000 people have been killed since the PKK took up its armed struggle for Kurdish self-rule in southeast Turkey in 1984. Washington, like Ankara, brands the PKK a terrorist group.

Turkey's Prime Minister Tayyip Erdogan will hold talks on Monday in Washington with U.S. President George W. Bush in what Ankara has portrayed as a last chance to avert a major strike.

DINNER

After meeting Erdogan and President Abdullah Gul in Ankara, Rice flew to Istanbul on Friday evening for talks with Iraqi Prime Minister Nuri al-Maliki before attending a dinner for the foreign ministers hosted by Babacan beside the Bosphorus.

Rice sat diagonally across from Iranian Foreign Minister Manouchehr Mottaki at the dinner, observers said, but appeared to avoid conversation with him.

U.S. officials say it is unlikely Rice will hold bilateral talks on Saturday about Iraq with Mottaki, whose country is at loggerheads with Washington over its nuclear programme.

Turkish-U.S. ties have come under strain following a resolution passed by a U.S. congressional committee last month that called the 1915 massacre of Armenians by Ottoman Turks a genocide. Turkey provides one of the main supply routes for U.S. troops in neighbouring Iraq.

Erdogan is under public pressure to act after dozens of soldiers died in recent PKK attacks, fuelling nationalist anger and sparking huge demonstrations across Turkey.

"The subject on our agenda is an operation, not a war. We hope that this operation will not be necessary," Erdogan said.

Rice said short-term measures included better intelligence sharing with the Turks and making it harder for the PKK, considered a terrorist organisation by the United States, the European Union and Turkey, to move around in northern Iraq.

Turkey plans economic sanctions that would target the PKK and groups providing them with support in northern Iraq, a move Rice said the United States could follow. (Additional reporting by Mussab Al-Khairalla in Istanbul and Evren Mesci in Ankara)

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