A Bulgarian ship has unloaded military helicopters, antiaircraft weapons and radar in the past 10 days, according to high-ranking diplomatic sources here.

The delivery indicates that Nicaragua's armed forces plan to escalate their fight against antigovernment guerrillas, the sources said. The United States, which has financed the guerrillas in the past via the CIA, has objected to Nicaragua's arms buildup and to its reliance on the Soviet Bloc for weapons.

A senior administration official with President Reagan in California said that U.S. officials also have "a strong suspicion" that Soviet-made MiG21 fighter jets are on their way to Nicaragua. Administration officials repeatedly have warned Nicaragua not to import advanced fighter jets, hinting at times that such a move could prompt U.S. military action.

Other officials, however, both in California and Washington, cautioned that intelligence reports about MiGs coming to Nicaragua are far from certain. One Pentagon official said, "I will believe it when I see it."

"It's a situation we're watching closely," said White House spokesman Larry Speakes. "If true, it could change the balance of power in this area." Later in the evening Speakes said that an East Bloc ship recently docked in Nicaragua, but he expressed doubt that it carried MiGs. "I would caution you not to jump to any conclusions about which kind of aircraft are aboard," he said. "It remains to be seen what is in the hold of the ship." Reagan may address the situation in a news conference in California Wednesday morning, White House officials said.

The Nicaraguan Embassy in Washington, reacting to American television reports, denied that MiGs were en route to Nicaragua.

A West European diplomat said yesterday that senior Nicaraguan officials had told him that no MiGs were being imported to avoid "provoking" the United States.

Critics of administration policy have said they fear the United States could use the arrival of new planes as a pretext to escalate the war against Managua's leftist Sandinista government now being waged by CIA-backed contra rebels. They have said they especially feared such an escalation after the U.S. election.

Senior administration officials said as early as last April that they viewed the covert war as a "holding action" to be stepped up in a second Reagan term.

The helicopters being unloaded would help Nicaraguan troops travel rapidly in the mountainous areas where the rebels are active. The antiaircraft materiel, which was said to be more sophisticated than anything Nicaragua has received previously, could help shoot down rebel supply planes that fly in from Honduras to drop ammunition and other equipment by parachute to guerrillas inside Nicaragua.

"More sophisticated arms are being brought in now. One would derive from this that they intend to step up the war against the contras," a knowledgeable foreign observer said. The guerrillas often are called contras as shorthand for "counterrevolutionaries." The new weapons were unloaded at the Caribbean port of El Bluff, said the sources, noting that a security cordon was established there Oct. 26. The diplomats here did not specify whether the helicopters were primarily designed for carrying troops or were outfitted as gunships, but diplomats had indicated that Nicaragua was to receive more Soviet-made Mi8s for ferrying troops.

Chief of state Daniel Ortega said Saturday that no planes were coming. A West European diplomat said yesterday that senior Nicaraguan officials had told him this was to avoid "provoking" the United States.

Nicaragua also is negotiating to obtain a less sophisticated Czech-made L39. A diplomat said that the United States "would object almost as much to L39s" as to MiGs because even L39s would "seriously upset the military balance in the region." Nicaragua's Air Force is the only branch of its armed forces that is inferior to that of its neighboring U.S.-backed rival, Honduras.