The agreement, worked out by Secretary of State Colin Powell and a Turkish leader, was designed to reduce any likelihood of Turkey moving military forces into northern Iraq.
At a joint news conference, Foreign Minister Abdullah Gul of Turkey said "new doors will open for us" in cooperating with the United States.
Powell said Turkey also had agreed to facilitate delivery of humanitarian assistance to Iraqi civilians and fuel supplies to coalition forces. He said enhanced military cooperation, such as evacuating wounded troops through Turkey or allowing coalition planes in distress to land in Turkey, was under consideration.
Powell said he hoped the new joint arrangement would be concluded within a week and that Iraqi Kurds would cooperate.
"We have the situation under control," he said. "There is no need for movement of troops across the border."
Turkey, with a large Kurdish population, is worried that Iraqi Kurds might declare independence and embolden separatists in Turkey, who have fought for autonomy in the southeast for 15 years.
Powell said the early warning system would provide for immediate consultation between Turkish and U.S. officials in the event of tension. At the end of the 1990-91 Gulf War, there was a surge of Kurdish refugees from Iraq into Turkey.
Powell's stop in Ankara was the first on a brief swing that seemed aimed at mending diplomatic fences torn in the run-up to war.
Turkey's refusal to permit use of its territory for U.S forces to invade Iraq strained U.S. relations with Ankara and caused the Bush administration to shelve a proposed $6 billion aid package.
After his talks in Turkey, Powell planned to fly to Belgrade for quick meetings, then head to Brussels, Belgium, for consultations with European Union officials. The EU was torn over the war, with Belgium, France and Germany resisting it.
With the war under way, there was potential for new tension. EU officials planned to tell Powell at a Thursday meeting that the United Nations must play the central role in rebuilding postwar Iraq.
EU spokeswoman Emma Udwin said Tuesday, "We believe the U.N. system has a unique capacity and experience in post-conflict states. The U.N. should play a central role during and after the crisis."
The Bush administration has hinted it would like the forces now fighting in Iraq to handle any transitional government after Saddam's defeat.
Powell also was meeting in Brussels with his Russian counterpart, Igor Ivanov. Russia strongly opposed the war and threatened to veto any Security Council resolution authorizing it.
Powell said Wednesday outside the Turkish foreign ministry that the U.S. military "worked around" Turkey's refusal to permit use of its territory. More than 1,000 U.S. troops parachuted into northern Iraq last week, accompanied by tanks and Bradley fighting vehicles.
The 4th infantry, which had planned to mount a northern front by invading via Turkey, has had its arrival in the Gulf delayed and is only now being deployed.
Gul, for his part, said "Turkey is within the coalition" and that overflights of Turkish territory by coalition combat aircraft would continue.
"The visit of Secretary Powell has strengthened our relations and helped to dispel all issues with regard to relations between the two countries," he said.
Powell, in similar warm terms, said U.S.-Turkey ties were strong and have endured for 50 years. "Turkey is an important member of the coalition against Saddam Hussein," he said.
Powell met with Gul, Prime Minister Recep Tayyip Erdogan and other Turkish officials over a six-hour period Wednesday.
About two-dozen students protested Powell's visit outside the Foreign Ministry, chanting "Murderer, murderer, get out of Turkey!" About five minutes before Powell arrived for the round of meetings, police detained all of the protesters.
Powell said the Pentagon was considering ways in which Turkey could assist in the war with Iraq beyond the use it has provided of its airspace for coalition combat aircraft.
Against that backdrop, the United States has started pulling some 50 warplanes out of Incirlik air base in southern Turkey after it became clear that Turkey would not allow them to be used in an Iraq war. The planes had patrolled northern Iraq since after the 1991 Gulf War.
Congress, meanwhile, is looking hard at President Bush's request for $1 billion in special assistance for Turkey. Powell said this is designed to deal with longtime economic woes.