In a bizarre bit of gunboat diplomacy, or perhaps because of a foul-up, the Soviet Union is sending a seventh guided missile patrol boat to Cuba, sources said yesterday.

A Soviet freighter, sources said, was towing the 205-ton gunboat of the Osa II class toward Cuba yesterday as Secretary of State Cyrus R. Vance and Soviet Ambassador Anatoliy F. Dobrynin were conferring in Washington on ways to defuse the issue of Soviet troops in Cuba.

This latest example of Soviet military aid to Fidel Castro is expected to add one more complication to the Vance-Dobrynin talks and provide fresh ammunition for politicians charging the Soviet Union with "bad faith" for stationing up to 3,000 combat troops in Cuba.

U.S. intelligence observed the Soviet freighter towing the gunboat out of the Black Sea at the end of August, learned the declared port of destination was Havana and has been watching ever since.

Although there is always a chance the freighter will be ordered to turn around, given the delicate state of Washington-Moscow relations, there was no evidence of this last night.

The newest delivery of an Osa patrol boat, though far from a blue-water threat to the United States, does represent part of what U.S. Navy leaders consider a significant modernization of Cuba's navy.

Retired British Capt. John Moore, editor of Jane's Fighting Ships, said in the latest edition of the book that Soviet aid has put Cuba in "a new naval league, especially in comparison to its Latin American neighbors.

The Osa en route to Cuba would be the seventh the Soviets have sent to the island during the last few years. With a speed of about 36 knots and armed with four SSN2 styx antiship missiles, the Osa boats are ideal for protecting Cuba's coastline or making hit-and-run strikes against navies of comparable size.

The Soviets also have sent Cuba 18 patrol boats larger than the Osa; 18 older Komar missile boats; seven landing craft and two diesel submarines as part of the modernization program.