US President George W Bush has issued a "very clear" warning to Turkey not to send troops into northern Iraq.
Turkey denied reports that its troops had entered Iraq
Mr Bush also said the US was working with Kurds in the region to avoid a conflagration in tensions with Turkey.
But Turkish MP Emin Shirin has told the BBC that his government is going to "judge what is necessary for the security of Turkey".
"[The Americans] are forgetting our concerns. First is immigration. We are talking about hundreds of thousands of people who could possibly immigrate. I don't think we can trust the Americans on this matter."
Our correspondent in south-east Turkey, Jonny Dymond, says that, despite denials from Ankara, there is little doubt Turkish forces have infringed the Iraqi border in recent days.
Small numbers of Turkish troops have operated in northern Iraq
since the 1990s, targeting Turkish Kurd rebel groups, but reports in the past week suggested that 1,500 extra troops had been deployed to prepare the way for an even bigger force.
Turkey fears the Kurds have ambitions to create an independent state in Iraq which could threaten its own south-eastern border.
"We are not forgetting that Mr Powell sent congratulations to the Kurdish Congress about a year ago," added Mr Shirin.
"About two weeks ago, [the Americans] were completely in agreement to move into Iraq together with us... All of a sudden after the Turkish parliament decided to deny the resolution to open the northern border... they find a Turkish presence in the north unacceptable."
Speaking to reporters at the White House on Sunday, Mr Bush said: "We've got more troops up north, and we're making it very clear to the Turks that we expect them not to come into northern Iraq."
"They know our policy, and it's a firm policy... and they know we're working with the Kurds to make sure there's not an incident that would cause there to be an excuse to go into northern Iraq."
Acknowledging a "limited belt" of Turkish soldiers along the border, Turkish Prime Minister Recep Tayyip Erdogan said they "aimed at stopping a possible influx of refugees... and prevent certain threats to our security".
"Turkey and the United States have reached agreement on all questions," he said in a nationwide television address.
US-Turkish relations have been severely strained by weeks of Turkish prevarication over whether to allow in thousands of US troops, who could invade Iraq from the north.
That idea now seems finally to have been abandoned.
A senior US defence official told the BBC that infantry equipment ships originally destined for Turkey - which have been awaiting instructions in the eastern Mediterranean - have now been ordered south to the Gulf.
Turkey has allowed the US an air corridor from which to launch air attacks - but not without enraging its strong anti-war movement at home.
In the TV address, Mr Erdogan said opening the airspace was "in Turkey's interests and is part of being an ally with the US".