Argentina's ruling junta went into session tonight, and while officials here did not mention reports that Buenos Aires was surrendering to British forces in the Falkland Islands, sources said the Argentine military position appeared untenable.

The military command did announce late today that an informal cease-fire prevailed in the Falklands' capital of Stanley after Argentine Gen. Mario Benjamin Menendez met with the commander of British forces that had reached the outskirts of the town. Later the military command said that Menendez and the British commander "were to have agreed on the conditions of a cease-fire," without confirming that the truce actually had been agreed to.

A military spokesman said tonight that Menendez was traveling to Buenos Aires to meet with the military junta and would return to the Falklands later. The spokesman, Capt. Enrique DeLeon, added in a terse briefing for reporters that "independent of the final result, the battle in Stanley is only one battle, and Argentina will never renounce its rights to the Malvinas," as Argentina calls the Falklands.

The Air Force commander in chief, Gen. Basilio Lami Dozo, returning from a trip to Argentina's southern coastal air bases tonight, told reporters that Argentina would not capitulate to Britain. He refused to comment on the results of the meeting between Menendez and the British commander.

"Today was the most difficult day in the South Atlantic," Lami Dozo said, "since April 2," when Argentina invaded the islands.

Buenos Aires remained quiet through the late afternoon as radio stations broadcast appeals for calm along with assurances that the public would be kept informed. President Leopoldo Galtieri's top aide, Gen. Hector Iglesias, told reporters outside the Foreign Ministry that the internal political front was "normal, absolutely peaceful."

Galtieri met for more than 2 1/2 hours during the day with the Navy commander in chief, Rear Adm. Jorge Anaya, as well as the chief of the joint chiefs of staffs and the Air Force chief of staff. Following Lami Dozo's hurried return from southern Argentina this evening, the full military junta went into session.

In addition, the Army command called a meeting tonight in Buenos Aires of all of the nation's generals who command brigades and divisions.

Foreign Minister Nicanor Costa Mendez told reporters here this morning that "we are working on all diplomatic fronts to continue the negotiations and to persuade and convince Great Britain of the obfuscatory and unjustified position they're in." But Foreign Ministry officials said Argentina had initiated no new diplomatic efforts to resolve the 10-week-old conflict by late today.

The military command said Gen. Menendez, the immediate commander of the reported 7,000 Argentine troops at Stanley, met with the British commander at 3 p.m. EDT after almost 18 hours of heavy fighting on all sides of the town.

In a series of six formal communiques issued throughout the day, the military command described British forces advancing on three fronts on Stanley beginning at 9:30 p.m. EDT last night. The high command previously had located the battlefront on a north-south line nine miles west of Stanley, in addition to British positions at Fitzroy, southwest of the capital.

Shortly before announcing the cease-fire, the military command said British forces had reached outlying settlements around Stanley. "Argentine troops, with great courage and resolve, continue to face an enemy that exceeds them in numbers, means, and technology," the communique said.

The report represented the first time that the military command had described British forces around Stanley as larger or better-equipped than the Argentine defenders. But newspaper reports, pegged to official sources, have stressed Britain's "technological" superiority in the last two days and attributed the advantage to military aid from the United States.

Galtieri and other high-ranking military leaders repeatedly have said in recent days that Stanley's loss would not end the conflict for Argentina, and that the military would continue to fight until it gained an acceptable solution. Military officials have said that Argentina could continue air attacks on the islands from its mainland bases, and Galtieri has maintained that Argentina will ally itself with any country willing to help it carry on the war, including the Soviet Union.

Sources here have said, however, that this policy of "total war" with Britain is opposed by powerful elements within the Argentine Army and Air Force, although nationalist civilian movements have endorsed it.

"They keep talking about what we are going to do, but they never mention how the British would respond," complained one Argentine Army official in a recent interview. "But we would soon have the British attacking La Plata an Argentine town near Buenos Aires , and we can't go on like that."

Lami Dozo suggested agreement with this point of view in an interview with the French newspaper Le Monde published here last weekend, saying that an Argentine triumph in the Falklands would be a "Pyrrhic victory" if it proved to be too costly. Last month, Lami Dozo said that while Stanley "was the most important bastion" of Argentina in the Falklands, "it is not the last bastion."