The Soviet Union, after a yearlong slowdown in deliveries of military hardware to the Nicaraguan government, has significantly stepped up arms shipments in recent weeks, transferring them through Cuba, administration officials said yesterday.

Officials said a U.S. SR71 spy plane, attempting to quantify the shipments in a mission over Cuba last Thursday, was targeted by a Cuban antiaircraft missile site but apparently did not draw fire.

According to recent intelligence reports, Soviet ships have unloaded crates of military equipment at the Cuban port of Mariel. The equipment, which appears to include trucks and T54 tanks, was later boarded on small Nicaraguan cargo vessels and delivered to the port of El Bluff in Nicaragua, administration officials said.

White House spokesman Larry Speakes told reporters that the Soviet arms shipments to the Sandinista regime have accelerated in recent weeks and represent a "serious increase" in weapons for use against antigovernment guerillas.

"It's important to note that the renewed military buildup coincides with the Sandinista government's crackdown on civil liberties and a military offensive against the democratic resistance in Nicaragua," he said.

Speakes said he was unable to provide "anything specific" about the buildup, but he repeated earlier charges that the leftist Sandinista government is strengthening itself militarily "far in excess of all its neighbors combined, that is far in excess of any defensive needs."

An administration official said the Soviet shipments to Nicaragua through Cuba are the first of any significance since Soviet freighters unloaded military equipment at Nicaraguan ports last fall. Despite U.S. concerns that those shipments included Soviet MiG fighter planes, analysts later viewed the weapons as largely defensive.

The official said precise quantities and types of equipment recently routed through Cuba remain unclear.

Nor is it clear why the supplies are being sent through Cuba rather than directly to Nicaragua, the official said. One possible reason, he said, is that Moscow is trying to obscure the source of the hardware to avoid controversy in the weeks before President Reagan's Nov. 19-20 summit with Soviet leader Mikhail Gorbachev.

An official said a SR71 Blackbird reconnaissance plane was flying over Cuba last Thursday when Cuban gunners targeted it with the fire-control radar of an SA2 antiaircraft missile site. Apparently no missile was fired, the official said.

The supersonic spy plane, equipped with high-power cameras, "was never in danger" and easily escaped, the official said. The SA2 is of Vietnam war vintage and was "no significant threat" to the U.S. aircraft, he said.

Cuba's official news agency reported the SR71 spy flight last Friday. Authorities in Havana organized a large demonstration outside the U.S. Interests section of the Swiss Embassy in protest.