Argentine Foreign Minister Nicanor Costa Mendez called on the Security Council today to impose a cease-fire in the Falklands and order a resumption of negotiations under the U.N. secretary general.

He argued that Britain had been the aggressor in the crisis, and affirmed once again that Argentine sovereignty over the islands is "unequivocal."

British Ambassador Sir Anthony Parsons replied that the cause of the conflict was the Argentine invasion on April 2, and its refusal to withdraw, as demanded by a council resolution adopted the following day. Once that cause was removed, he said, the hostilities would be ended.

With that exchange, the council completed its five-day debate on the Falklands after it made some progress toward adopting a resolution to resume U.N. peace efforts in the crisis. A compromise resolution circulating tonight was given some chance of acceptance.

The amended resolution--in less specific language than two original texts submitted by Ireland and Japan--asks Secretary General Javier Perez de Cuellar to immediately contact Britain and Argentina in efforts to arrange a cease-fire and to report back to the council as soon as possible, but not later than seven days.

Neither of the original drafts had appeared viable.

The Irish text had urged both sides to accept a 72-hour truce, during which time Perez de Cuellar would seek a permanent cease-fire and then a final settlement.

The British, however, have made clear that it would be prepared to veto any such proposal. The only cease-fire Britain could accept, Western diplomats reported, is one that commits Argentine forces to withdraw.

The second proposal had come from Japan and asked the secretary general to resume his mediation "on the basis of his previous efforts," with a view to "achieving the earliest possible cessation of hostilities" and a peaceful settlement that would include Argentine withdrawal.

Such a resolution would provide the illusion of council action but leave the parties still at war.

Several Third World nations, among them Zaire and Uganda, teamed up tonight on the proposal to give the secretary general seven days to work out cease-fire terms with the parties.

A British source displayed some interest with a cryptic statement that "a lot can happen in seven days." The Argentines indicated they needed instructions from home, and further consideration was put off until Wednesday.

Argentine officials said Costa Mendez would leave New York Wednesday to attend a meeting of the Organization of American States that Argentina has asked to be convened Thursday in Washington.

Costa Mendez joined the foreign ministers of Venezuela, Panama and Nicaragua--all of whom showed up to support Argentina in the council debate--in a joint declaration calling on the United Nations to halt the fighting and deploring the extension of sanctions against Argentina by the European Community, with the "honorable" exceptions of Ireland and Italy.

The most notable aspect of the statement was the rapprochement between Argentina and Nicaragua. Just two months ago the Sandinista regime had accused the Argentine junta of training and supplying guerrillas operating against it. And Washington was counting on Argentine support against leftist elements in Central America.

In his speech to the council today, Costa Mendez sought to make the case that the British aggression lay in its colonial possession of the Falklands for 149 years. Colonialism, he said, is intrinsically a crime, an act of force, and "permanent aggression."

Afterward, asked by reporters if Argentina would surrender, Costa Mendez shot back, "What? Translate that word." Asked again whether Argentina would give up, the foreign minister ended the colloquy by saying "that is not in my dictionary."