The Soviet leadership Monday rejected a bid by a predominantly Armenian region to secede from the republic of Azerbaijan in a bitter territorial dispute that has sparked five months of unrest in the Caucasus.

"The presidium of the Supreme Soviet of the USSR confirmed today that Nagorno-Karabakh belongs to Azerbaijan," the official Soviet news agency Tass said.The decision at the special session was unanimous, even though Armenia - which has supported the effort by the region of Nogorno-Karabakh to separate from Azerbaijan - is represented on the Presidium.

At the same time, the Presidium "made provision for measures that would ensure the region's real autonomy as well as unconditional fulfillment of a comprehensive program for its social economic and culutural development," Tass said. The Presidium is the government's highest executive body.

Nagorno-Karabakh has been part of the mostly Moslem Soviet republic since 1923, although three-quarters of its 160,000 inhabitants are ethnic Armenians, who are predominantly Christian.

On July 12, the legislature of the Delaware-size district voted to secede from Azerbaijan and join neighboring Armenia, but Azerbaijan's government voided the decision hours later, calling it illegal.

The 1977 Soviet Constitution requires that any of the nation's 15 constituent republics agree before any change can be made in its borders.

Both Nagorno-Karabakh and Armenia, which has voted to accept the region, asked the central Soviet leadership to intervene in the dispute, which has kept the Caucasus on the boil with ethnic tensions since February and has triggered strikes that have cost more than $100 million.

The Presidium, headed by President Andrei A. Gromyko, has more than 30 members and includes top government and Communist Party officials including Mikhail S. Gorba-chev, the party's general secretary, and top leaders from all 15 republics.

Tass said representatives of Armenia, Azerbaijan and Nagorno-Karabakh would first deliver reports to the Presidium and that members of the executive body would take part in the debate. It gave no estimate of how long the meeting might last or when a decision could be expected.

Because of the constitutional provision that gives Azerbaijan an effective veto over any change in Nagorno-Karabakh's status, it was uncertain what moves the Presidium might take.

Tass said Gromyko opened the session by noting that the situation in Nagorno-Karabakh had worsened since the Presidium last discussed the issue on March 23, when it rejected as "intolerable" the Armenian campaign for annexation.