Turkish forces have crossed into Iraq in what seems to be their biggest operation against Kurdish rebels sheltering there in years.

Up to 10,000 troops may be involved in the operation which, according to the army, began on Thursday evening after an air and artillery bombardment.

Turkey promised its force would "return home in the shortest time possible after its goals have been achieved".

The US military says it believes the operation is "of limited duration".

Turkey has promised to do everything possible to avoid "collateral damage to innocent civilians or Kurdish infrastructure," said Rear Adm Gregory Smith, a US spokesman in Iraq.

But another US official quoted by Reuters news agency appeared to express concern about the scale of the Turkish operation.

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It was "not the greatest news" and marked "a whole new level", Deputy Assistant Secretary of State Matthew Bryza said in Brussels

'Unusual timing'

"A cross-border ground operation backed by the Air Force was launched at 1900 [1700 GMT on Thursday]," the military said.

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