The Soviet Union is believed by western diplomats to have ordered its East European allies to curtail sharply their high-level political contacts with the West in the aftermath of the Afghanistan crisis.

British and West German diplomats say they believe the Kremlin wants to head off any situation in which western countries could drive a wedge between Moscow and the rest of communist Eastern Europe over the controversial Soviet invasion of Afghanistan.

The Soviet attack is known to be unpopular among many of Moscow's allies.It threatens to bring a return to the Cold War with the United States and thus possibly to jeopardize the steadily expanding trade, credit and contact with the West that the East Europeans have come to depend on. It also could bind them even closer to the Soviet Union.

As evidence supporting their view that the Kremlin has given orders to curtail the most visible high-level contacts, the western diplomats cite a strong of sudden "indefinite postponements" by eastern governments.

Soon after the Soviet thrust into afghanistan last Dec. 27, communist East Germany announced indefinite deferral of a previously proposed summit meeting between West Germany Chancellor Helmut Schmidt and East German leader Erich Honecker.

Late in January, Hungary postponed a scheduled visit to West Germany of the Hungarian foreign minister and the Czechoslovakian government rescinded an invitation to West German Foreign Minister Hans-Dietrich Genscher to visit Prague.

Early in February, the Hungarians also suddenly postponed a visit to the United States of the president of the Hungarian parliament because of "unexpected" political duties in Budapest.

West European officials and some U.S. State Department officials say the East Europeans privately have made it clear, especially to the West Germans, that the cancellations involved Soviet pressure -- that they were, in effect, made on Moscow's orders.

Another State Department specialist, however, said he was not certain that specific Kremlin orders were behind the rash of postponements. He believes that East European governments, all of which are especially good at both sensing what the Kremlin wants and protecting their maneuvering room with Moscow, may have decided to do these things on their own until the crisis cools.

The postponements thus far have centered heavily on West Germany and the United States.

Moscow consistently has put the heaviest pressure on West Germany throughout a string of tense East-West situations in recent months because Bonn is viewed both as highly influential in the West and as highly vulnerable to Soviet pressure in Berlin and in its relations with East Germany.

The West Germans are of considerable importance to Moscow's Warsaw Pact allies because Bonn, in most cases, is their largest western trading partner.

Carter administration policy toward Eastern Europe has tended to differentiate between countries that generally have good relations with the United States -- such as Poland, Hungary and Romania -- and those such as Czechoslovakia and East Germany that are much more tied to Moscow's hard-line policies.

In combination, the United States and West Germany are the two key western countries that Moscow undoubtedly would like to deprive of any opportunities to exploit differences over the wisdom of the Afghanistan invasion.

Romania, traditionally that maverick within the Warsaw Pact and frequently independent of Moscow on foreign policy questions, is the only country thus far clearly resisting any Kremlin order on contacts.

British Foreign Secretary Lord Carrington will visit Bucharest next week and West German chancellor-candidate Franz Josef Strauss will follow him to the Romanian capital.

Of the major western powers, France has been the least critical of the Soviet action. This month the secretary general of the French Foreign Ministry, Bruno De Leusse, visited Moscow. De Leusse, a former ambassador to Moscow, is the fourth-highest ranking official within the Foreign Ministry.However, French officials point out that De Leusse also is a "nonpolitical" appointee.

Both U.S. and West European officials say that lower-level contacts continue between East and West, including a variety of discussions on trade and the upcoming 35-nation conference to review the 1975 Helsinki agreements on European security and cooperation.

U.S. officials say the East Europeans have privately made clear their interest in pursuing their relations with this country in trade, arms control and various exchange programs, and that the United States has made clear its mutual interest.

At the same time, the U.S. officials said, Washington has warned Moscow's allies about any attempt to undermine newly imposed U.S. restrictions on grain or high technology exports to the Soviet Union stemming from the Afghanistan crisis.

The officials claim the East Europeans have been responsive to this warning, understand it and reportedly accept the U.S. position.