The United States and Turkey will sign an agreement today to allow three vital U.S. bases in Turkey to stay open, but talks on the scope of their operations will continue, Western diplomatic sources said yesterday.

The current agreement on strategic U.S. military bases in Turkey is set to expire today. The two countries still disagree on whether the United States can use the bases for intelligence gathering and military operations in the Middle East and against the Soviet Union, the sources said.

The bases come within the framework of the North Atlantic Treaty Organization (NATO), of which the United States and Turkey are members, and are aimed mainly against any Soviet threat.

But political convulsions in Iran, the Middle East and the Soviet invasion of Afghanistan have thrust the bases, particularly two scanning stations and a big tactical aircraft base, into the limelight. So the United States does not want that distinction spelled out in the new agreement, according to the sources.

They said the United States believes there might be other obligations, such as those arising from U.N. membership, mutual U.S. Turkish security or even world peace, that the bases could be used to respond to.

The Turks also have demanded joint control of the bases, but according to the sources, the United States is hesitant to grant that since it would set a precedent for U.S. bases around the world.