An incursion across the border by Turkish troops has long been in the offing, although it was not immediately clear yesterday how big the operation was or how long it would last. But it is a serious escalation of Turkey's battle against the rebels after weeks of aerial assaults on PKK bases in the mountains that straddle the Turkish-Iraqi border.
The Turkish government said it had alerted the Bush administration to the operation in advance, but the news still caused alarm in Washington and Brussels. The operation is a blow to the US, which last year made a series of efforts to persuade Turkey not to carry out a large-scale ground incursion.
Washington urged caution on Ankara yesterday, but played down its previous warnings that such an operation could destabilise northern Iraq and the surrounding region.
"We urge the Turkish government to limit their operations to precise targeting of the PKK, to limit the scope and duration of their operations," said Scott Stanzel, a spokesman for President George W. Bush.
Javier Solana, the EU's foreign policy chief, said the incursion was "not the best response" to Turkey's PKK problem.
Recep Tayyip Erdogan, Turkey's prime minister, said: "We are in a new stage in our fight with terrorism." He added that the only aim was to protect Turkish citizens.
He got opposition backing for the incursion, which is likely to be popular with the public and with Turkey's gung-ho media.
Analysts in Ankara said the timing of the operation was as significant as its scale. They said clashes between the PKK and the Turkish military usually peaked in the spring, and the incursion appeared to be an attempt by the military to destroy as much rebel infrastructure as it could in a surprise attack.
A spokesman for the Kurdistan Regional Government said the offensive had occurred in mountainous border regions, and that he was unaware of any casualties. The KRG has consistently said it considers the PKK a Turkish domestic problem, with a political rather than a military solution, and that it opposes any violation of Iraqi sovereignty.
Some Turkish reports suggested that up to 10,000 troops had crossed into Iraq, although a US military official told Reuters that "a few hundred" were involved.
The Turkish military did not say how many troops took part but said they would leave "as soon as planned goals are achieved". The operation would "prevent the region [northern Iraq] from becoming a permanent and safe base for the terrorists and will contribute to Iraq's stability and internal peace".
Turkey has been fighting the PKK for three decades, with the loss of thousands of lives, mainly in south-eastern Turkey, where many of the country's estimated 12m Kurdish citizens live. But the military has failed to deliver a knock-out blow. It may take time for an assessment to emerge of whether the latest assault had scored a strategic victory, diplomats in Ankara said.