The Soviet Union warned Great Britain tonight that it regards the British imposition of an exclusion zone around the disputed Falkland Islands as "unlawful."

"The British government continues expanding the zone of combat operations in the Atlantic Ocean, arbitrarily proclaiming vast expanses of high seas closed to ships and aircraft of other countries," the government news agency Tass said, quoting an official statement.

"These actions clearly contradict the 1958 convention on the high seas and, consequently, are regarded by the Soviet side as unlawful."

The statement was read to British Ambassador Curtis Keeble, who was summoned to the Foreign Ministry. It was Moscow's first diplomatic initiative in the Falklands crisis after weeks of propaganda attacks on Britain and the United States.

Diplomatic observers said the statement appeared to be a warning to Britain against interfering with the freedom of navigation of Soviet fishing and commercial vessels in the South Atlantic. Argentina is one of the main suppliers of wheat and meat to the Soviet Union.

There also was speculation here that Moscow see the crisis as entering a critical stage and that the warning may have been designed to have a political impact on public opinion in Argentina as well as the rest of Latin America.

Ambassador Keeble was quoted as having described the Soviet statement as "ill-founded." British officials here refused to comment on his exchange with Vladimir Suslov, a Foreign Ministry department head.

The Soviet government described as "provocative" what it called British efforts to seek a "diplomatic cover" for its actions by inventing "the aleged Soviet Union's 'involvement' in the AngloArgentine conflict over the Falkland (Malvinas) Islands."

The statement continued: "Facts show that the Soviet side does not give the slightest grounds for such allegations and this is well known in London."

Tass said keeble was told such assertions by Britain "are absolutely unacceptable and cannot be regarded by the Soviet side in any other way than as provocative and not promoting the maintenance of the normal climate in Soviet-British relations."

So far, the Soviet media has reported Britain's military preparations in the crisis extensively. While expressing strong sympathy for Argentina, the Soviet have been careful not to endorse Argentina's seizure of the islands April 2.

In general, Moscow has been viewing the crisis as welcome source of friction between the United States and Latin America and a possible prelude to the collapse of the government of British Prime Minister Margaret Thatcher.

Soviet President Leonid Brezhnev said in a speech last week that the crisis demonstrated "the imperialist nature" of American policy toward Latin America but called for a negotiated settlement.

There have been no hints that Moscow was considering the possibility of a direct involvement in the dispute other than to seek political and propaganda advantages by opposing "colonialism" and portraying the United States as having "betrayed" a Latin ally at a crucial time despite treaty commitments to come to its aid.

Today's statement did not include any specific warnings although it implicitly raised the question of Soviet navigation in the Soviet navigation in the South Atlantic. The Soviets bought more than $3 billion worth of Argentine goods last year, including 85 percent of all Argentine grain exports. They are also importing about 100,000 tons of Argentine beef annually.

Relations between the Kremlin and the Argentine junta have developed in the wake of then president Carter's partial grain embargo against Moscow. In just over two years, commercial ties between the two countries have dramatically expanded with Soviets reportedly buying more than 10 million tons of grain annually.

It is believed here that the Soviets have been monitoring British naval movements by satellites and possibly have information that leads them to think the British may be about to launch an invasion of the Falklands.

Soviet sources have said privately that Moscow expects to reap long-term benefits from the current conflict once it is settled as having shown itself to be on the side of Latin Americans.

"Five years from now when everybody has forgotten where the Falkland Islands are located, the Latin Americans will remember that the United States had sided against them," one source said.