In moves that pose serious problems for President Carter's attempts to curtail international arms traffic, two Latin American countries - Mexico and Guatemala - are seeking to buy U.S. F5 jet fighters worth a total of $180 million.

The secret requests involve Mexico's desire to purchase 26 of the supersonic jets at a cost of roughly $150 million and a separate, but parallel bid by Guatemala for six F5s worth $30 million.

The Mexican request is regarded as an especially sensitive matter by Carter administration officials because Washington's ultimate decision on the jet sales - whether positive or negative - could do serious damage to some high-priority U.S. foreign policy objectives.

The impact of the military aircraft requests is the subject of a brisk, behind-the-scenes debate within the U.S. national security community.

Selling the planes to Mexico, some governmental critics point out, would run directly counter to the president's announced policy objective of setting an example for other armsproduing nations by reducing U.S. foreign military sales, particularly in Third World areas.

The sale of Sophisticated jets to Mexico would put considerable tarnish on the crenibility of the policy. The Defense Department now estimates that arms sales and grants will rise from $11.2 billion in fiscal 1977 to $13.2 billion in fiscal 1978.

It also would trigger attacks from liberal U.S. and Mexican crities, who would view it both as a spur to Latin American militarism and a strategically unnecessary extravagance that the financially hard-pressed Mexican government cannot afford.

On the other side, though, U.S. officials are known to fear that rejecting the bid would offend the Mexican government and jeopardize the special relationship that Carter has worked so hard to establish with President Jose Lopez Portillo.

In this view, turning the Mexicans down could cause an estrangement that would adversely affect U.S. efforts to enlist Mexico's cooperation in such sensitive areas as resolving the illegal immigration problem, obtaining greatly increased supplies of Mexican natural gas and crude oil for the United States and combating the flow of illegal heroin across the border.

The Guatemalan request, although much smaller in scope, also has ticklish implications. There, the main problem involves Guatemala's claim to sovereignty over neighboring Belize and fears in Central America and the Caribbean that any advanced weapons obtained by Guatemala might someday be used in an attempt to annex Belize territory by force.

In addition, although Guatemala's president, Gen. Kjell Laugerud Garcia, was constitutionally elected, the country is dominated by the armed forces, which frequently have been accused of practicing political and social repression. That raises a conflict between the Guatemalan request and the Carter administration's policy of generally denying arms assistance to countries with human'rights problems.

In trying to pick its way through these problems and reach a decision on the requests, the Carter administration has resorted to a variety of delaying tactics in the month and a half since the requests were made. Specifically, reliable sources say, Washington has stalled by asking the two Latin governments for more information about how they plan to finance the proposed purchases.

However, the sources add, some divisions already have become apparent among U.S. officials, particularly in regard to the Mexican request.

According to the sources, Defense Department experts, while conceding that the Mexican air force's arsenal of planes is outmoded, argue that Mexico's defense needs do not require a sophisticated plane like the F5. In their view, the sources say, the request should be rejected, or Mexico should be offered some less expensive, subsonic type of military jet fighter.

However, the sources add, some officials within the State Department lean to the view that Mexico, if turned down by Washington, would simply turn to another country like France or Britain, with a cooresponding loss of U.S. influence in Mexico. Accordingly, the sources reportedly favor meeting at least part of the request to avoid further troubling U.S. Mexican relations, which have been strained by a quarrel over the price the United States should pay for Mexican gas.

As to Guatemala, the sources say, there is agreement among U.S. officials that the planes normally could not be sold as long as the Belize controversy remains unresolved.

That, the sources, add, is an especially critical consideration at this time because of the strong support that Belize, the former British Honduras, has among the English-speaking countries of the Caribben. Lately these countries have been increasingly angered by reports that Britain, with the backing of Washington, has been pressuring Belize to appease Guatemala by surrendering a token part of its territory.

However, the sources add, there also is an awareness in the administration that it would be very difficult to turn down Guatemala if a favorable decision is made on Mexico's request. That, the sources say, would expose the United States to charges of overtly favoring the larger countries of Latin America at the expense of its smaller ones.

At the root of these problems is an airplane, manufactured by the Northrop Corp that has periodically caused friction in U.S.-Latin American relations ever since its development in the late 1950s as a lightweight, easily flown and simply maintained fighter tailored to the needs of the world's smaller air forces.

The original F5, which was commonly known as the Freedom Fighter, has been replaced, in the 1970s, by a variety of newer versions that cost about $3 million each in their basic, stripped down form. With special modigications to meet the needs of purchasing countries and with auxiliary ground and training equipment, the cost can rise to $5 million each, and beyond.

Although the United States has supplied F5s to a number of Third World air foces, particularly in the Middle East, it tired, during the 1960s, to keep supersonic fighters out of Latin America by banning sales of F5s in that region.

That embargo was broken in 1967 when Peru, frustrated in its attempts to purchase the F5, turned to France and bought 16 of that country's supersonic Mirage jets. The United States then began competing actively with France and Britain for supersonic jet sales to latin American forces.

At present, three Latin countries - Brazil, Chilo and Venezuela - have some version of the F5. All three countries, plus Argentina and Colombia, also have supersonic Mirages or British Hawker Hunters; and Cuba's air force is built around Soviet supersonic MIGs.

What appeared to mark a new clamp on U.S. sales was signaled by Carter when he announced his arms sale policy last May 19. He said his administration intended to reduce substantially the sale of U.S. military hardware and would put the burden of persuasion in any future arms deals on those who favor them rather than those who oppose them.

According to the sources, one question mystifying U.S. officials is why Mexico has asked for the F5s at this time. In contrast to most Latin American countries, Mexico has not spent much money on its armed forces in recent years, and Lopez Portillo has been struggling desperately to resolve a number of pressing financial problems that have plagued Mexico for more than a year.

Some U.S. officials, the sources say have speculated that the Mexican military, after decades of quiescence and submission to civilian authority, may be getting restive and that Lopez Portillo regards modernization of the badly outdated Mexican air force as a pacifying gesture.