The United States yesterday informed Poland and the Soviet Union of its "serious concern" over the Polish military crackdown, but President Reagan and other senior U.S. officials avoided any suggestion of U.S. intervention.

Reagan, arriving back at the White House after a weekend at Camp David, said the United States, "along with the whole free world," has made clear to the Soviet Union "how seriously we would view any interference in Poland."

But the president cautiously refused to answer reporters' questions about what actions the United States might take. He said only: "We're monitoring the situation. Beyond that, I can't have any comment."

Earlier, in Brussels, Secretary of State Alexander M. Haig Jr. made a longer but similarly measured statement in which he disclosed that "a senior Polish foreign official" had assured the United States and other western nations that "reforms will continue" despite the military takeover.

Haig said the United States "will watch very carefully" whether this promise will be kept and declared that "the political experiment under way in Poland should be allowed to proceed unimpeded."

The secretary broke off a scheduled trip to the Middle East, Asia and Africa to remain temporarily in Brussels, and late yesterday he announced he will return to Washington today to give attention to the Polish situation. Deputy Secretary William P. Clark has been in charge of the State Department during his absence.

The secretary said the Polish developments, which began early yesterday morning as he was preparing to leave Brussels for Israel, had come as a surprise to him and America's European allies.

In fact, it was learned yesterday, at a National Security Council meeting Friday, the administration, unaware that the Polish crisis was about to boil over, decided to authorize $100 million in new emergency feed grain credits for Poland. Earlier this month, Poland asked the United States to expedite $200 million of a requested $740 million in credits to buy agricultural products.

A high-level Polish delegation led by Deputy Premier Zbigniew Madej was here last week to meet with Vice President Bush and other officials about the U.S. assistance.

Senior U.S. officials said the visitors gave no indication they were aware that a confrontation between the Polish government and the dissident labor movement Solidarity was imminent. Although he did not mention Friday's NSC decision specifically, Haig made clear yesterday that the whole question of U.S. financial help is likely to be reconsidered very carefully in the light of the weekend's events.

Haig also said U.S. concern was being conveyed to Soviet and Polish authorities.

In Washington, later in the day, Alexander A. Bessmertnykh, minister counselor of the Soviet embassy, was called to the State Department for a meeting with Walter J. Stoessel Jr., under secretary for political affairs; Polish Ambassador Romuald Spasowski had a similar session with John D. Scanlan, deputy assistant secretary for European affairs.

Haig and officials here admitted that interruptions in communications and travel to and from Poland were hampering U.S. efforts to get a clear picture of internal events there. As a result, although it was apparent a lot of behind-the-scenes attention was being given to the Polish crisis, the administration sought to say as little possible about what might happen next.

The main questions of concern to U.S. policymakers were what the response of the Polish people might be when the new work week begins today and the related issue of open Soviet intervention which could trigger a crisis in East-West relations.

Complicating the situation was the fact that the crackdown came not through an overt Soviet move into Poland but was carried out instead by the Polish military, which can boast of strong nationalistic credentials.

For that reason, it was not immediately clear whether the West is confronting what amounts to an indirect, Soviet-orchestrated intervention, which Haig has said would be met by a "collective western response," or a situation that will be popularly viewed in Western Europe as an internal Polish matter.

Until that is clarified, it is not possible to predict how the events of the weekend will affect such matters as efforts to improve the general state of Soviet relations with the West, the recently started U.S.-Soviet talks on reducing the number of medium-range missiles in Europe, attempts by both sides to influence the growing anti-nuclear movement in Western Europe and moves by the West to ease tensions in Poland through economic help.

Haig hinted that if the developments in Poland lead to full-scale repression, that could end the flow from the West of food and financial support for that debt-ridden, increasingly destitute nation.

He volunteered to reporters that his telephone consultations yesterday with allied foreign ministers involved "especially those with responsibilities for assistance levels to Poland." He added that a continuation of U.S. food aid "would have to be considered very carefully in the light of events which will unfold in the hours ahead."

Treasury Secretary Donald T. Regan, interviewed by CBS television, warned that the latest turns in the Polish situation "could have a serious economic aspect." He said, "Even without the political crisis today, Poland would have an economic crisis tomorrow."

Regan said Poland has an external debt of $26 billion, of which $16 billion is owed to foreign banks and the rest to various governments, and is unable to generate sufficient foreign exchange to pay even the interest on these loans.

"If Poland defaults and the banks have to write off this debt, there are going to be a lot of international banks put in a very precarious position," Regan warned. He added that while not many U.S. banks are involved, a sizable number are in other western countries.