President Reagan said today the Soviet Union has been "closed-mouthed" about the nuclear reactor accident at Chernobyl, and White House officials said two offers of U.S. technical, medical and firefighting assistance have been made without a response from the Kremlin.

"Well, they're usually a little closed-mouthed about these things, and this is no exception," Reagan told reporters as he began a meeting here with Indonesian President Suharto.

White House spokesman Larry Speakes announced today that a Soviet Embassy official, Victor Isakov, had brought a message to the State Department Tuesday afternoon that was described as coming from Soviet leader Mikhail Gorbachev. Speakes said the message provided details of the accident and expressed appreciation for U.S. offers of help but did not request assistance.

This message followed a meeting between Assistant Secretary of State Rozanne L. Ridgway and the Soviet Embassy's second-ranking official, Oleg M. Solokov. In both meetings, U.S. officials offered assistance. But Speakes said today the United States has received no response.

Reagan's first public comments on the nuclear accident reflected the private irritation of many U.S. officials that the Soviets had delayed informing the world of the accident.

In his briefing today, Speakes said it is a longstanding tradition for nations to provide information about transboundary threats to other countries and insisted that the Soviets have a responsibility to notify other nations about the accident "in a timely fashion." He said the Soviets had not done so.

Speakes accused Soviet authorities of maintaining a "close hold" on information about the accident.

Speakes said the United States learned of the accident, which began Friday, from a Tass news agency account as the president was flying Monday to Indonesia.

Reagan, asked about the possibility that radiation could spread to the United States, said he had received "a map from some of our people here who are experts in that field" and that these experts said "by the time it reached our country, it would have been so widely dissipated that it wouldn't represent any health threat."

Speakes said the radiation from the site has "stabilized" but that a "quite large" radioactive air mass continues to be "unsettled." It moved to the northwest, then to the south and now is moving east over the Soviet Union. He described the radiation as stretching over portions of eastern and northern Europe, the Soviet Union and the Arctic Basin.

He said "above permissible norms" of radiation have been detected but that the levels do not appear to necessitate special protection.

Speakes said administration does not expect "any significant health effects, if indeed it does reach the United States." He compared the radiation emitted from the accident to the average amount thrown into the atmosphere by the kind of above-ground nuclear tests conducted by the United States in the late 1950s and early 1960s.

Speakes announced that satellite photographs have shown a second heat source at the reactor site but said U.S. officials cannot confirm there has been a fire at a second reactor.

Speakes said the Soviets had told the United States that the accident began in the power block room of Unit 4 of the Chernobyl power station, that there was a reactor leak and a fire. He said the Soviets have also told U.S. officials of a "partial evacuation" of the area adjacent to the reactor complex, but that the administration has no firm information about casualties or danger to Americans in the area.

As long as the reactor fire continues, he said, the smoke will carry radiation into the atmopshere, adding that the fire could burn for weeks. He said the concentrations of radiation could depend on which the way the prevailing winds take it -- if the radiation is carried over the arctic route toward the western United States it could come in higher concentrations than if carried east over the Soviet Union.