Defense Secretary Caspar W. Weinberger, reflecting a growing concern among senior administration officials, said yesterday that the Soviet Union has enhanced its ability to invade Poland during the last 48 hours and that he considers the situation there to be "very serious."

Testifying before the Senate Budget Committee, Weinberger declined to specify what Soviet steps are of most concern to the administration, and he stopped well short of predicting a Soviet invasion. But he said of the overall situation in Poland:

"It's not improving in terms of time at all," he said. "It's getting worse as the particular hours have gone by in the last few days. As the last few days have gone by, the situation has worsened from my point of view."

Administration officials, expanding on Weinberger's comments, said the chief cause of American concern was the failure of the Soviet Union to reduce the level of military activity around Poland in the wake of the decision by leaders of the independent Solidarity union movement there to call off plans for a general strike earlier this week.

While the United States expected a decline in the level of military readiness near the Polish border after the strike was called off, sources said the Soviets appear instead to have put at least one additional division on alert in the area.

"Instead of winding down they appear to be winding up," one official said.

Officials said they are also concerned about a step up in the levels of Soviet Propaganda regarding Poland and a decided hardening of the Soviet line, including recent attacks on Polish communist officials. The fact that moderate elements in the Polish communist hierarchy are now being criticized was taken as a potentially ominous sign of Soviet impatience with how the internal situation in Poland is being handled by the communist government there.

While Weinberger was testifying, other administration officials were grappling with the economic aspects of the Polish crisis in a series of meetings that ended with the announcement that the United States will sell 50,000 tons of surplus dairy products to Poland at a reduced price.

Officials said the dairy products -- 30,000 tons of dried milk and 20,000 tons of butter -- have a current world market price of $73 million and are being sold to Poland for $50 million. As a further concession to the economically embattled Poles, the United States has agreed to accept Polish currency as payment rather than demanding that the commodities be paid for in dollars, officials said.

Unlike the dollar and other currencies used in world trade, Polish currency has no purchasing power outside of Poland.

The dairy products sale was announced at the White House by Vice President Bush after his meeting with Mieczyslaw Jagielski, Poland's deputy prime minister, who also met yesterday with Secretary of State Alexander M. Haig Jr. and other administration officials.

Bush said the United States was also considering "as a matter of some urgency" other matters raised by Jagielski, an apparent reference to Poland's request for a multibillion-dollar economic aid program from the western powers that would include deferrals of loan payments due this year.

Western economic assistance to Poland is to be discussed by officials of more than a dozen countries in Paris next week. Announcement of the dairy products sale in connection with the Jagielski visit was described by officials as a gesture of U.S. good will while it and its allies consider the larger Polish aid request.

Soviet military activity around Poland has been of deep concern to administration officials recently. Much of this activity has taken place in connection with a previously announced Warsaw Pact exercise known as Soyuz 81.

Last week, with events apparently headed for a critical juncture over the general strike decision in Poland, the military exercise was extended beyond its scheduled termination date as a show of force by the Soviets.

The decision to continue the military exercises brought expressions of concern from Haig and Weinberger last week. Yesterday, the concern appeared to have deepened as, with the immediate threat of a general strike passed, the military exercises continued.

At the State Department, spokesman William Dyess said that while the level of military activity around Poland remains "unusually high" there are no signs that Soviet troops have begun to move toward Poland.

Dyess also said that the U.S. position remains that an invasion of Poland is "neither imminent nor inevitable."