Turkey's parliament has given permission for the government to launch military operations into Iraq in pursuit of Kurdish rebels.
The US fears any incursion could destabilise the region
The vote was taken in defiance of pressure from the US and Iraq, which have called on Turkey for restraint.
Turkish Prime Minister Recep Tayyip Erdogan has said the motion does not mean a military operation is imminent.
But he said Turkey needed to be able to respond to a recent rise in bomb attacks blamed on PKK rebels from Iraq.
Turkish MPs backed him overwhelmingly, by 507 votes to 19.
As the vote was being counted, US President George W Bush strongly urged Turkey, a key ally, not to carry out the threatened action.
He said Washington was "making it clear to Turkey it is not in their interest to send more troops in... there is a better way to deal with the issue".
The recent deaths of 13 Turkish soldiers in an ambush blamed on the PKK has put the government under renewed pressure to respond with force.
What Turkey wants now is a convincing response from allies and neighbours, says the BBC's Sarah Rainsford in Ankara.
Turkey has been calling for help in tackling the PKK for months. Now it hopes the rest of the world will realise it is serious, our correspondent says.
But the US and Iraq fear any incursion could destabilise the only relatively calm region of Iraq.
Iraqi Prime Minister Nouri Maliki phoned the Turkish prime minister before the vote, saying he was "absolutely determined" to remove the PKK from Iraq and pleading for more time, according to Turkey's Anatolia news agency.
Iraqi President Jalal Talabani, an ethnic Kurd, urged Turkey not to make an incursion, but also called on the PKK "to end the so-called military activity".
The autonomous Kurdish regional government in northern Iraq warned Turkish MPs that any intervention would be "illegal". It has denied providing the PKK with any help.
The rebels themselves said they would meet force with force.
The chief of the PKK's executive council, Murat Karayilan, told the Kurdish Hawlati newspaper: "Thousands of PKK guerrillas are on standby to fight Turkish army forces."
However Syrian President Bashar Assad, visiting Turkey, said he supported the country's right to take the action "against terrorism and terrorist activities".
President Bush, speaking during a press conference, criticised the US Congress for jeopardising US relations with Turkey with a planned vote to recognise the mass killing of Armenians in Ottoman times as genocide.
"One thing Congress should not be doing is sorting out the historical record of the Ottoman Empire," he said.
Although a congressional committee has supported the motion, its chances of passing a full vote appear to be waning.
Key Democrats in the US House of Representatives have joined Republicans to warn that US strategic interests could be damaged by the largely symbolic resolution.