A new crisis has exploded in Iraq after Turkish troops, supported by attack planes and Cobra helicopters, yesterday launched a major ground offensive into Iraqi Kurdistan.

The invading Turkish soldiers are in pursuit of Kurdish guerrillas hiding in the mountains. They are seeking to destroy the camps of the PKK (Kurdistan Workers' Party) along the border between Turkey and Iraqi Kurdistan. "Thousands of troops have crossed the border and thousands more are waiting at the border to join them if necessary," said a Turkish military source.

"There are severe clashes," said Ahmed Danees, the head of foreign relations for the PKK. "Two Turkish soldiers have been killed and eight wounded. There are no PKK casualties." Turkish television said that the number of Turkish troops involved was between 3,000 and 10,000, and they had moved 16 miles inside Iraq.

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But the escalating Turkish attacks are destabilising the Kurdish region of Iraq which is the one peaceful part of the country and has visibly benefited from the US invasion.

The Iraqi Kurds are America's closest allies in Iraq and the only Iraqi community to support fully the US occupation. The president of the autonomous Kurdistan Regional Government, Massoud Barzani, said recently he felt let down by the failure of the Iraqi government in Baghdad to stop Turkish bombing raids on Iraqi territory.

The incursion is embarrassing for the US, which tried to avert it, because the American military provides intelligence to the Turkish armed forces about the location of the camps of Turkish Kurd fighters. Immediately before the operation began, the Turkish Prime Minister, Tayyip Erdogan, called President George Bush to warn him.

The US and the Iraqi government are eager to play down the extent of the invasion. Rear Admiral Gregory Smith, a US spokesman for Iraq, said: "We understand [it] is an operation of limited duration to specifically target PKK terrorists in that region." The Iraqi Foreign minister, Hoshyar Zebari, claimed that only a few hundred Turkish troops were in Iraq.

But since last year Turkey has succeeded, by making limited incursions into Kurdistan, in establishing a de facto right to intervene militarily in Kurdistan whenever it feels like it.

Many Iraqi Kurdish leaders are convinced that a hidden aim of the Turkish attack is to undermine the Kurdish region, which enjoys autonomous rights close to statehood. Ankara has always seen the semi-independence of Iraqi Kurdistan, and the Kurds' claim to the oil city of Kirkuk, as providing a dangerous example for Kurds in Turkey who are also demanding autonomy.

Many Turkish companies carrying out construction contracts in the region have already left. And businesses that remain are frightened that Ankara will close Iraqi Kurdistan's lifeline over the Harbour Bridge into Turkey.

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During the 1990s the Turkish army carried out repeated attacks in Iraqi Kurdistan with the tacit permission of Saddam Hussein, but this is the first significant offensive since the US invasion of 2003. "A land operation is a whole new level," said the US Deputy Assistant Secretary of State Matthew Bryza, adding that the incursion was "not the greatest news".

The Turkish army is unlikely to do much damage to the PKK, which has some 2,500 fighters hidden in a mountainous area that has few roads, with snow drifts making tracks impassable.

The Turkish ground offensive was preceded by bombing. "We were certain yesterday after this bombing that a military operation would take place and we got ready for it," said Mr Danees, adding that bombing and artillery had destroyed three bridges on the Iraq-Turkish border as well as a PKK cemetery.

Another reason why Turkey has launched its offensive now has as much to do with Turkish internal politics as it does with any threat posed by the PKK. The PKK launched a military struggle on behalf of the Kurdish minority in eastern Turkey in 1984 which lasted until the PKK's leader Abdullah Ocalan was seized in Kenya in 1999 and later put on trial in Turkey. The PKK has been losing support ever since among the Turkish Kurds, but at the end of last year it escalated guerrilla attacks, killing some 40 Turkish soldiers.

Limited though the PKK's military activity has been, the Turkish army has used it to bolster its waning political strength. For its part, the mildly Islamic government of Mr Erdogan is frightened of being outflanked by jingoistic nationalists supporting the military. Mr Erdogan has pointed out that previous Turkish army incursions into Kurdistan in the 1990s all failed to dislodge the PKK.

The area which the Turkish army has entered in Iraqi Kurdistan is mostly desolate, with broken terrain in which bands of guerrillas can take refuge. The PKK says it has left its former bases and broken up into small units. The main bases of the PKK are along Iraq's border with Iran, notably in the Kandil mountains, to the south of where the Turkish troops entered. At this time of year the villagers, many of them herders and shepherds, leave their houses and live in the towns in the plain below the mountains until the snow melts.

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