Brussels — The recent closing of the Tehran bureau of the official Soviet news agency Tass is the most tangible indication of a steady deterioration in Iran's relations with the Soviet Union.
Iranian sources say the first sign of divergence came last July when Moscow radio described the first Iranian offensive within Iraq as ''conflicting with the United Nations resolution calling for a cease-fire and with the goodwill displayed by the Iraqi government.''
Previously Moscow had stuck to what it called a position of strict neutrality between Iran and Iraq. But, since the outbreak of the war the Iraqis have often complained that the Soviet government was refusing to provide them with much-needed military hardware despite a friendship treaty. The Soviets were also said to be encouraging Syria, Libya, and North Korea to sell weapons and ammunition to Tehran.
A few days after the outbreak of war, Muhammad Ali Rajai, the then Iranian prime minister, even claimed to have received an offer of military assistance from the Soviet ambassador in Tehran. Moscow denied it, but in November 1980 the Soviets signed with Tehran a transit agreement that helped Iran face the economic blockade imposed by the West and the closure of its most important southern ports. Moreover, the Persian-speaking radio station based in Baku repeatedly criticized Iraq for ''being a tool in the hands of America.''
Last summer a turn in the Soviet stance toward revolutionary Iran was also reflected in a Radio Baku editorial criticizing the war against the Kurdish rebels. ''There is an ethnic problem in Iran,'' said the commentator, ''and the Islamic government should recognize it.''
The Iranians were quick to react. A few days later the head of the revolutionary guards, Mohsen Rezaie, claimed that the Iraqi Army was again resupplied with Soviet equipment. He stressed that ''at least one of their brigades has been equipped with highly sophisticated T-76 tanks.''
While repeatedly denying that Iran has ever leaned toward the Soviet Union, Minister of Foreign Affairs Ali Akbar Vellayati stepped up his attacks against Soviet intervention in Afghanistan. The tension climaxed when the Tass correspondent in Tehran did not report on the killing of dozens of Iranian civilians by a Soviet-made missile in the southern town of Dezful.
Now, endorsing a thesis often developed by their Western colleagues, Iranian editorialists explain that the ''Soviet Union had calculated playing a mediating role between us and Iraq which would have strengthened its position in the area of the Persian Gulf.''
After a period of development, economic relations between the two countries have become stagnant. Iran has yet to reopen its gas pipeline to Soviet Union. Instead it is discussing with Turkey the possibility of building a pipeline that would take its gas to Europe.
The cooling of the relations with Moscow was followed by a crackdown on the communist Tudeh Party whose leaders, including Secretary General Noureddine Kianouri, were arrested in early February. The news published first by an anticommunist morning paper was later confirmed by the government. Some see this as an indication that Mr. Kianouri and his friends were arrested by a group of revolutionary guards acting on their own.
Meanwhile, Tehran's revolutionary prosecutorhad announced a few months ago that ''he would prove the links between the communist party and Soviet Union.'' Although the communist leaders were charged with spying for the KGB and using false passports, little is known about the exact reasons for the arrests.
Reports circulating in Western Europe tell of a document on Soviet activities in Tehran that could have been passed to the Iranians through mysterious intermediaries. This document is said to have been drafted in Britain with the help of a former senior diplomat of the Soviet Embassy in Tehran who defected to the West a few months ago.
Tudeh members are very pessimistic about the outcome of the trial of their leaders.
''The party hasn't been banned yet,'' says a Tudeh member in Europe. ''We believe that the Iranian government is split on the issue.''
This opinion is shared by the Soviet press which claims that ''the struggle between progressives and CIA sponsored counterrevolutionaries is raging in Tehran.''
This rising anti-Soviet and anticommunist tide does not lessen anti-American rhetoric. Ayatollah Khomeini never misses an occasion to castigate Washington which is still described as the main source of evil on earth.