The Soviet Union has begun flying a number of military transport planes into Poland, deepening concern among American officials that Moscow is close to a critical decision on whether to intervene directly in Poland's continuing turmoil, intelligence officials disclosed last night.

Officials said, however, that they did not know the purpose of the flights and had made no assessments that the flights were a first step in a Soviet invasion of Poland.

"We're not sure what's in them, how much is in them and what it means, but there is no assessment that this is a prelude to an invasion," one senior official said.

The planes were being flown to airfields on existing Soviet military bases inside Poland, analysts said. The Soviet Union has long maintained two divisions inside Poland.

Several senior officials said there was no evidence that Soviet divisions that hae been poised near the Polish border for several months were moving into Poland.

But the flights of the aircraft into Poland, coupled with intelligence data showing that the Soviets, according to one official, have 12 to 15 divisions ready to move into Poland "at a moment's notice," clearly increased American concern over the situation in Eastern Europe and brought forth fresh warnings about the grave consequences of a Soviet invasion.

U.S. intelligence officials said that for the Soviet Union to invade Poland on the same scale as Czechoslovakia in 1968 it would have to continue its current deployment and buildup for another week or 10 days.

The heightened level of American concern was reflected in a State Department statement, read to reporters by spokesman William J. Dyess, that for the first time since last December openly suggested that an invasion could take place at any moment.

One worrisome sign was hard evidence that the Kremlin had recently activated some Soviet reservists, possibly to replace Warsaw Pact forces that might by sympathetic in the independent labor movement in Poland, according to intelligence analysts.

Although the Soviets could launch some small-scale military raids into Poland, these analysts said that a full-scale invasion would take about a week to set in motion. "No date has been set," one high government official said last night in summing up the best evidence of Soviet intentions.

Administration officials said later that the State Department statement was issued in the hope of influencing the Soviet decision on intervention in Poland. Both the level of recent military activity around Poland and the sudden criticism of Polish communist authorities by Moscow this week were among the factors leading intelligence analysts to believe the decision to invade or not is about to be made, officials said.

"Soviet military activities around Poland continue at unusual levels despite the fact that tensions within Poland have been reduced," Dyess said.

He said the United States is "watching the situation closely" and is also "concerned with tendentious and distorted Soviet press commentary which appears to be aimed at providing justification for possible Soviet action," a reference to the criticism of Polish communist authorities.

"We do not believe that a Soviet invasion in inevitable, but the Soviets are capable of moving at any time," Dyess said. He added that "such an unjustified action would have the gravest consequences for East-West relations."

For several weeks, and as late as Thursday, the U.S. position has been that a Soviet invasion of Poland is "neither imminent nor inevitable." The omission of the word "imminent" from yesterday's statement was seen as a sign that the United States now fears that a Soviet troop movement into Poland could be near.

Asked about this, Dyess said, "Soviet reaction time [to invade] has been reduced as a result of increased readiness. . . . The Soviets are capable of moving at any time."

In response to another question, he said the situation around Poland is "perhaps more serious" than it was last December, when the Soviets first massed troops along the Polish border, threatening an invasion to quell labor strife.

Taken together, these statements suggested a steadily mounting level of American concern over the situation in Poland, which Defense Secretary Caspar W. Weinberger on Thursday termed "very serious."

At a lunch with reporters and editors of The Washington Post yesterday, Weinberger said Soviet troop activity around Poland has continued and on a larger scale than expected. Officially, the Soviets have said these troop movements have been part of a previously announced Warsaw Pact exercise known as Soyuz 81. But Weinberger said yesterday that it is "increasingly unlikely" that the continued troop activities are connected with the Soyuz 81 exercise.

Weinberger said Thursday that the United States was "taking steps" in response to the Soviet activity. Sources said this referred to consultations with allies on possible political, economic and diplomatic responses to an invasion and not to military preparations.

A Soviet move into Poland in the next few days would not only find President Reagan in the hospital recovering from the wound he suffered in an assassination attempt Monday, it would also find Weinberger and Secretary of State Alexander M. Haig Jr. out of the country.

Weinberger left late last night on a week-long trip to Western Europe. Haig left Washington a few hours before him on a 10-day mission to the Middle East and Europe.

Dyess scoffed at suggestions that given the heightened tension around Poland Haig should have postponed his trip. "Our feeling is that the trip to the Middle East is very important and should go ahead," he said.