By Tabassum Zakaria

ISTANBUL, March 24 (Reuters) - U.S. Vice President Dick Cheney on Monday told Turkey that the United States supported its fight against Kurdish rebels in northern Iraq, but wanted to ensure it did not add to political tensions in Iraq, a senior U.S. administration official said.

Cheney also discussed Turkey's role as part of the NATO force in Afghanistan but received no firm commitment that Ankara would send more troops or increase support, the official told reporters travelling with Cheney to Istanbul.

The United States is trying to smooth tensions between two key allies, Iraq and Turkey, over Turkey's incursions into northern Iraq to fight Kurdish rebels known as the PKK.

"The United States has certainly been supportive of Turkey in that fight against the PKK," the U.S. official said on condition of anonymity.

"We've worked hard with the Turks as well as with the Iraqis to try and figure out how to get at the PKK problem in as productive a way as possible," the official said.

That meant helping Turkey and Iraq fight the PKK while being sensitive to Iraq's delicate political and security situation, and "trying hard to avoid any problems that would add to the existing stresses on the Iraqi political balance," he said.

Cheney started a nine-day trip to the Middle East with a stop in Iraq that included a trip to Arbil, where he met Kurdish leader Masoud Barzani last week and discussed the PKK among other issues.

Turkish-U.S. relations have been rocky in recent years but improved after Washington shared intelligence during a Turkish ground offensive against Kurdish rebels in northern Iraq.

Turkey is a NATO ally for Washington and a key conduit for U.S. troops in Iraq and Afghanistan.

Several small protests greeted Cheney's Turkey visit to Ankara. In one, some 150 people chanted slogans against the Iraq war and U.S. imperialism, and called on Turkey to reject a U.S. request to send more Turkish troops to Afghanistan.

The United States is pressing NATO allies to provide more support for Afghanistan, and it will be a key issue next month at the NATO summit in Bucharest.

Cheney was told during his talks in Ankara that Turkey was going to stay engaged in Afghanistan, but he received no immediate commitments about doing more, the U.S. official said.

Turkey's Chief of General Staff General Yasar Buyukanit, who met Cheney on Monday, later reiterated Turkey's opposition to sending more troops to Afghanistan when the army was fighting the PKK elsewhere. (Additional reporting by Selcuk Gokoluk and Zerin Elci in Ankara; Editing by Jon Boyle)

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