Sooner or later, the Falkland Islands crisis is expected to come back to the United Nations.
Britain won the first round against Argentina at the United Nations Security Council on April 3 immediately following Argentina's military occupation of the Falkland Islands. At that time the UN overwhelmingly condemned Argentina's invasion.
But Argentina could well win the second round, according to senior diplomats here.
Argentina has now taken a first step toward reconvening the Security Council. Panama, a member of the council, obviously at the suggestion of Buenos Aires, has asked for urgent informal talks among council members.
It has not yet set a date for a formal meeting of the council. This move is interpreted here as meaning that Argentina will not get the UN involved as long as the Haig mission has not collapsed. Argentina also wants to remind Washington that it has the option of resorting to the UN and it hopes this will help persuade Secretary of State Alexander M. Haig Jr. to stay involved and be more sympathetic to Argentina's point of view.
A letter addressed by Panama to the President of the Security Council indicates the case, from a juridical point of view, that Argentina intends to present to the council.
Basically, Argentina's argument would rest on the fact that if the British Navy shoots at Argentina vessels, the roles would be reversed. This time Argentina would be the underdog and Britain would appear to be the aggressor.
Moreover, the resolution passed by the Council on April 3 not only asked Argentina to withdraw from the Falkland Islands. More significantly for Argentina, it also called for an immediate cessation of hostilities. Were the British to use force, they would stand in violation of the resolution.
As the Argentines interpret Article 51 of the UN Charter, self-defense can be invoked by a country against an aggressor only up to the moment when the Security Council has taken the necessary measures to keep the peace.
In other words, Argentina hopes to present its occupation of the Falklands Islands as a fait accompli legally in conformity with:
* The Inter-American Treaty of Mutual Assistance and the collective security system of the OAS charter.
* The Monroe Doctrine (''The American continent cannot be considered grounds for colonization by European powers'').
* The declaration of the nonaligned summit meeting in Havana of 1979 which recognizes Argentine sovereignty over the Falkland Islands.
* Finally, Argentina plans to denounce the presence in the South Atlantic of British nuclear subs as a violation of the Tlatelolco Treaty, prohibiting the presence of nuclear weapons in Latin America.
Some of these arguments are considered by legal experts here to be highly debatable or even downright specious. They may, however, carry political weight. Soviet Union, which abstained on April 3 is expected to back Argentina this time and the nonaligned members of the council who supported Britain as the victim the last time may well lean toward Argentina the next time.
''Britain's Navy would seem to be acting not in self-defense but in seeking reprisals,'' says a nonaligned ambassador.
Informed sources here believe that the council could take either of two steps to try to resolve the dispute, should Haig's mission fail.
1. It could ask the UN secretary-general to use his good offices and seek a compromise between Britain and Argentina.
2. It could ask a delegation of three Security Council members to shuttle between London and Buenos Aires in much the same fashion as Mr. Haig and try to settle the crisis peacefully.