Turkey has agreed to let the US use its airspace in its campaign against Iraq with immediate effect.
The US needs Turkish airspace to lift troops to northern Iraq
Defence Minister Vecdi Gonul told reporters the agreement "is in Turkey's interests".
He announced the decision after talks with Prime Minister Recep Tayyip Erdogan and senior military and civilian officials.
It is not clear if the US has agreed that, in return, Turkish troops can enter Iraq.
Friday's agreement came after earlier talks between Turkey and US failed to resolve their differences.
Most Turks are against the war
Turkey wants to send troops into Kurdish-controlled northern Iraq to stem a potential tide of refugees and prevent the Kurds forming a separate state.
Before Friday's deal was announced, US Secretary of State Colin Powell had said: "At the moment we do not see any need for any Turkish incursions into northern Iraq."
The US has said its forces will manage the refugee problem, and take control of the Kirkuk and Mosul oilfields just south of the Kurdish-controlled area.
The US also fears Turkish forces could become involved in fighting against local Kurdish forces.
The Turkish parliament had given consent for the US flights on Thursday.
Large numbers of Turkish troops are reported to have massed on the border.
Press reports say some units have already crossed over.
The US originally asked Turkey to grant permission for 62,000 of its troops to use the country as a launch pad for an attack on Iraq.
Turkey would have received a multi-billion-dollar compensation package in return.
US officials say the financial package has now been withdrawn.
Correspondents say Friday's agreement will make it easier for the US to fly supplies into Iraq.
Before Friday's announcement, Turkish officials had said that any agreement for US aircraft to overfly the country would not extend to allowing them to refuel at Turkish airbases.