Turkish ground troops have entered northern Iraq in a drive against Kurdish separatists of the Kurdish Workers Party which Ankara accuses of using bases in Iraq to launch attacks against its forces in southeastern Turkey. Although Kurdish rebels say two Turkish troops have been killed in the fighting and another eight have been wounded, Turkey has not commented on those reports.

In a statement released on its web page, the Turkish military says its forces entered northern Iraq on Thursday evening after carrying out air and artillery strikes against suspected PKK positions. The military has not released figures on the size of the incursion but according to Turkish media, 10,000 troops are taking part.

Turkey has been massing forces and commandos on its border with Iraq over the last few weeks. But with the region in the grip of winter, the operation was widely expected to take place a few weeks from now. The incursion is being seen as a preemptive strike against the Kurdish rebels, who usually begin a spring offensive with the melting of winter snows.

"Land force operation is compulsory, not optional because PKK activities increasing in the spring time," said retired Turkish General Armagan Kuloglu. "Otherwise they will move again very soon."

The PKK is believed to have as many as 3,000 fighters in bases across the mountainous semi-autonomous Kurdish enclave in northern Iraq.

Turkey has repeatedly called on the U.S. and Iraqi governments to move against the rebels, who have been fighting for autonomy in Turkey's mainly Kurdish southeast since 1984. But Washington has limited its involvement to providing intelligence on the PKK to Ankara.

Turkey's patience ran out last year following a number of PKK attacks which killed some 50 Turkish soldiers and civilians. Over the last few months, Turkish jets have launched air strikes against PKK bases.

With Turkish troops entering Iraq, Ankara is expected to face intense diplomatic pressure.

Washington has expressed concern about the risk of growing tensions between Turkey and the the government in Baghdad, both U.S. allies.

U.S. Senator John Kerry, visiting Ankara, stressed Turkey's importance.

"Let me say today, on behalf of the American people, for the relationship we enjoy today, and for the help that the Turkish people and the Turkish government is providing in Iraq, Afghanistan and Pakistan," said Kerry.

U.S. officials have confirmed the raid, and a U.S. military spokesman in Iraq said Washington understands the operation is of limited duration. The U.S. is encouraging Turkey to resolve the PKK problem using diplomacy and close coordination with the government of Iraq.

Aware of U.S. concerns, the Turkish military said it will return home in the shortest time possible after its goals have been achieved.

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