The United States is considering an increase in its supply of ammunition and military spare parts to Jordan to help that Mideast country counter the buildup of Syrian forces near its border, the State Department said yesterday.

Jordan asked for the extra help after the border tensions began to increase over the weekend.

Department spokesman John Trattner, while stressing that "no decisions have been made," noted that "Jordan is a friend whose security is important to us." The Carter administration, he said, is consulting with King Hussein's government and doesn't "exclude the provision of some further deliveries of spares and ammunition."

Although Trattner refused to specify what kind of equipment might be involved, reliable sources said any increased sales to Jordan would involve relatively modest amounts and would be limited to small-arms ammunition and parts for vehicles and Jordan's American-made F5 jet fighters.

"We are not going to do anything that would upset the basic balance of power in the region," one official said. The United States, he pointed out, has to walk a delicate line that means not alarming Israel, also a neighbor of Jordan, that Hussein is becoming too strong militarily and that also will not permit Jordan to use beefed-up military supplies to aid Iraq in its war against Iran.

On the other hand, the official addeed, Jordan has been threatened by Syria's massing of troops near the border. Reports from the Middle East have put the Syrian buildup at as many as 50,000 men, although U.S. intelligence estimates believes the correct number is closer to 30,000.

The Syrian regime of President Hafez Assad has conducted a long rivalry with Iraq for leadership of the radical faction of the Arab world, and Syria has been hostile to Iraq's efforts to win prestige by wresting control of disputed territory from Iran. Although U.S. officials do not believe Syria is seeking an actual conflict with Jordan, they do consider its troop buildup a muscle-flexing exercise aimed at intimidating Hussein, who has been the most outspoken supporter of Iraq in the Persian Gulf war.

As a result, U.S. officials said, any additional military sales to Jordan should be seen not as a "tilt" toward Hussein but as an attempt to keep the Mideast conflict from spreading by deeterring Syria from precipitate actions. m

In that respect, the officials added, the United States has been working quietly with a number of other countries in the region, including Saudi Arabia, to urge restraint on the Syrians. The Saudis have made eenormous contributions from their oil wealth to both Syria and Jordan.

The prinicpal U.S. military deal with Jordan is an agreement reached last spring to sell Hussein 100 M60A3 tanks with advanced night-sights for approximately $160 million. However, delivery on the tanks is not scheduled to begin until 1982, and Trattner said yesterday there are no plans to accelerate that timetable.

The United States also sells Jordan artillery and antiaircraft missiles, radar equipment and small arms and and has beeen replacing gradually the Jordanian air force's aging F5 fighters with more advanced versions of the same plane.

During the 1980 fiscal year, which ended Sept. 30, the United States provided Jordan with $60 million in economic supporting funds. In the fiscal year in 1981 budget, which has not yet been approved by Congress, the administration has proposed military sales credits to Jordan of $50 million in economic support funds.