ANKARA, Turkey (AP) _ Russia, Turkey and the United States have offered two warring republics in the former Soviet Union a plan to end their five-year conflict over a mountain enclave, Turkey's foreign minister said Monday.

Details of the plan presented to Azerbaijan and Armenia were not released. But Foreign Minister Hikmet Cetin said it aims to ''secure an effective cease- fire and confidence-building measures.''

More than 3,000 people have been killed in the battle over Nagorno- Karabakh, a mountain region populated largely by Christian Armenians that is located entirely inside Muslim Azerbaijan.

Armenian forces have recently made gains on the battlefield, opening a corridor between the enclave and Armenia.

In Washington, the State Department said it was pleased with the proposal and urged the warring parties to accept it.

Aram Abramian, the Armenian presidential spokesman, said Armenia had received the document and would respond ''in the coming days.''

There was no immediate comment from Azerbaijani officials.

Cetin said the plan was devised during talks among the United States, Russia and Turkey in Moscow on April 29 and 30. The consultations were proposed by Turkey a week earlier during the funeral of President Turgut Ozal of Turkey.

Turkey has close ethnic, cultural and economic ties with Azerbaijan.

In response to military gains by Armenian forces last month, Turkey sealed its border with Armenia, halted civilian and aid flights through its airspace and stopped grain shipments to Armenia. Cetin said Monday the measures were still in force.