Secretary of State Alexander M. Haig Jr. said yesterday the Reagan administration is "watching with incresing concern" the levels of sophisticated weapons being shipped into Nicaragua and "the high level of manpower" being assigned to the Nicaraguan armed forces.

"We see no threat [to Nicaragua] that justifies increases of this size," Haig said. "We will be watching closely the levels of arms that have arrived and that are expected to arrive."

Haig, answering questions at a State Department meeting for editors and broadcasters from around the country, was asked about a report in yesterday's Washington Post saying the United States has received intelligence reports that Soviet T55 tanks may have been sent secretly into Nicaragua.

He refused to comment directly on the report. But he did contend that high levels of arms "of a worrisome nature" are continuing to flow into Nicaragua from Cuba.

Some of this arms flow, Haig said, continues to be diverted to leftist guerrillas fighting the U.S.-backed government in neighboring El Salvador. Although he cited the big buildup of Nicaraguan forces being engaged in by the leftist-oriented, revolutionary government there, Haig did not specify whether any of the alleged flow is part of a long-rumored plan to equip the Nicaraguans with Soviet weaponry including tanks and MIG jet fighters.

Earlier, however, department spokesman Dean Fischer confirmed that the United States has received intelligence reports that Soviet tanks may have been shipped into Nicaragua and that additional tanks are in Cuba awaiting delivery. While Fischer said the reports have not been confirmed, he added that the presence of such Soviet weapons would pose "serious problems" for other Central American countries.

"As Nicaragua adds military equipment to its already substantial arsenal, tensions do inevitably increase," Fischer said. "We would consider the presence of heavy Soviet armor or aircraft to pose serious problems for Nicaragua's neighbors.

Daniel Ortega, the head of Nicaragua's revolutionary junta, has told The Washington Post the reports his country intends to obtain Soviet tanks and jets are "totally unfounded." However, the Nicaraguan government, which won power in 1979 after a bloody civil war, has made clear its intention to build a powerful military force, and there have been reports that the goal is for 50,000 men and women.

The Nicaraguans already are believed to have more than 20,000 people on military duty or undergoing training. The junta has justified the buildup on the grounds that Nicaragua must protect itself against hostile, military-dominated rightist regimes in Honduras, El Salvador and Guatemala and anti-revolutionary Nicaraguan exiles centered in Honduras.