The BBC report also says this:

After Turkish broadcaster NTV reported that 10,000 soldiers had crossed the border on Thursday, a Turkish military source confirmed for Reuters news agency that two brigades had been deployed.

A brigade numbers up to 5,000 troops, the agency notes.

The operation's timing is unusual as the mountainous border area is still covered with heavy snow, the BBC's Sarah Rainsford reports from Istanbul.

Nor have there have been any major PKK attacks inside Turkey for some time, she adds.

More than 30,000 people have been killed since the PKK began fighting for a Kurdish homeland in south-eastern Turkey in 1984.

Baghdad confusion

As news of the operation broke, Iraqi Foreign Minister Hoshyar Zebari told the BBC he was not aware of any incursion since speaking to BBC correspondent Jim Muir late on Thursday.

The minister, himself a Kurd from northern Iraq, said he had been in touch with people in the north and in Istanbul, as well as being in close contact with the US.

However, the office of Turkish President Abdullah Gul said the leader had phoned his Iraqi counterpart, Jalal Talabani, about the operation on Thursday evening.

"During the call, our president conveyed information on the objective of the ground operation," Mr Gul's office said.

Rear Adm Smith said the US supported Turkey's "right to defend itself from the terrorist activities of the PKK" and encouraged it to "co-ordinate closely with the government of Iraq".