Turkey and the United States today signed a five-year agreement for the joint operation of 12 defense installations in this NATO-member country.

In exchange for continued military cooperation with the United States Turkey was understood to expect an estimated $2.5 billion in U.S. aid over the next five years to revitalize the Turkish economy and modernize its armed forces.

The signing ceremony capped 15 months of negotiations and became effective immediately. It appeared to signal a major improvement in Turkish-American relations, which have been strained for six years as a result of the 1974 Turkish invasion of Cyprus and subsequent U.S. arms embargo.

Although Prime Minister Suleyman Demirel's government will submit the agreement to the Turkish parliament for ratification, the United States considers the pact an administrative agreement, which does not need to be ratified by the Senate.

The accord, renewable at yearly intervals after the initial five-year period, was signed by Turkish Foreign Minister Hayrettin Erkmen, and U.S. Ambassador James W. Spain.

The 12 installations include vital intelligence gathering stations at Sinop on the Black Sea and Pirinclik in the south. Also covered are a seismological base at Belbasi, near Ankara, a navigational station on the Marmara Sea and the huge NATO base at Incirlik in southern Turkey. The other seven facilities are communications stations scattered around the country.

A Turkish Foreign Ministry statement stressed that the bases were Turkish facilities and that the United States was being allowed to "participate" in their operations.

It said the military cooperation agreement came strictly "within the sphere of the NATO region and NATO obligations" -- a clear indication that the bases were not to be used for U.S. military operations in the Middle East not sanctioned by the North Atlantic alliance.

The United States lifted its arms embargo against Turkey in 1978 and Turkey subsequently reopened joint military facilities that had been closed in retaliation for the embargo.

Although no specific figures for U.S. aid to Turkey were cited in the defense agreement, for the 1981 fiscal year the United States has already agreed to provide $200 million in economic aid to Ankara and $250 million in military aid.