The State Department reported yesterday that South Yemen military forces have begun to withdraw from positions in North Yemen, easing an international mini-crisis which had threatened to take on great power dimensions.

Spokesman Tom Reston said, however, that the United States will go ahead with the promised supply of $520 million in jet aircraft, tanks and other weapons "because we think it is needed for the defense of the territorial integrity of North Yemen." Reston also said he knew of no change in the orders to a naval task force involving the U.S. aircraft carrier Constellation to patrol the Arabian Sea near the Yemen conflict.

News agency dispatches from the area quoted Syrian Gen. Ibrahim Younis, head of an Arab League truce supervision team, as saying that Both sides had pulled out of battle zones along their joint border. The operation began on Saturday after Arab League negotiation ended three weeks of fighting, and was reported nearing completion yesterday.

Younis was quoted as saying that the two sides have also reached an understanding on measures to eliminate tension and prevent further clashes.

U.S. officials said that if the ceasefire and withdrawal holds, a planned airlift of war materiel to North Yemen might be changed to slower means of delivery. However, they added that no decision will be taken until the situation is stabilized.

President Carter on March 5 ordered strong United States military and diplomatic efforts to assist North Yemen amid reports that this is where the United States had decided to "draw the line" against the Soviet Union and Cuba. About 1,000 Soviet advisers and 800 Cuban advisers and large amounts of Soviet war materiel are reported in South Yemen.

The area is particularly sensitive because both Yemen states border on Saudi Arabia, the world's number one oil exporting nation. U.S. efforts to back North Yemen were designed in part as a gesture to the Saudis.