The Reagan administration yesterday relaxed restrictions on trade with Syria and South Yemen--both on the U.S. government's formal list of nations supporting international terrorism--by allowing them to buy civilian aircraft if they promise not to use them for military purposes.

The move, coming a few days after the administration took the controversial step of removing Iraq from the terrorism-supporter list, is expected to provoke strong criticism in Congress.

Syria, one of the most vehemently anti-Israeli Arab countries, has a long record of supporting terrorist activity against the Jewish state, and South Yemen has been described repeatedly by Secretary of State Alexander M. Haig Jr. as a "Soviet surrogate" in the Middle East.

In addition, the administration's decision comes at a time when U.S. officials have been seeking to calm Israeli concern that the United States is tilting toward the Arab side in the Mideast conflict because of a desire to sell increasing amounts of weaponry and military equipment to Arab countries.

Congressional sources said that, despite these considerations, a letter was sent to Congress yesterday by Bohdan Denysyk, assistant secretary of commerce for export administration. It said the Reagan administration will permit American manufacturers to sell "civil aircraft for use by regularly scheduled airlines based in Syria and the People's Democratic Republic of Yemen for which assurances against military use have been submitted to the Department of Commerce."

The 1979 Export Administration Act requires that Commerce issue a special license when sales of equipment with a potential military use are proposed for countries on the terrorism list. If Commerce decides to grant the license, it must notify Congress, a requirement that in the past has resulted in congressional moves to block some of these sales.

The administration's action will exempt aircraft sales to Syria and South Yemen from both the licensing and congressional-notification requirements. However, unlike Iraq, which is being relieved of all restrictions applying to countries in this category, Syria and South Yemen will remain on the terrorism list, and the requirements will continue in force where sales of other military-related equipment are concerned.

The other countries on the list are Libya and Cuba, which was added last week. Administration officials said yesterday that the exemptions for Syria and South Yemen had the approval of the State Department, even though Haig, starting with his first news conference after taking office, has said repeatedly that the administration regards the fight against international terrorism as one of its highest priorities.

State Department sources said privately that the decision was based primarily on the economic consideration that U.S. aircraft manufacturers have run into increasing competition from the European-made Airbus in their efforts to sell planes to Arab customers.

After reviewing the matter, these sources said, the administration concluded that Syria and South Yemen have not misused aircraft belonging to their scheduled civilian airlines, that such equipment has little practical military application and that U.S. firms were being hurt unnecessarily.

However, that argument ignored the fact that when Libya was allowed to buy Boeing 727s, it used them to transport troops and equipment to Uganda to help deposed dictator Idi Amin in his unsuccessful effort to retain power through repression.

Congressional sources predicted that the administration's justification would also draw heavy fire on Capitol Hill on the grounds that it will increase Israeli suspicions of U.S. intentions and undermine the credibility of the administration's antiterrorism efforts.