Nicaragua went before the Security Council again today to hammer home its case that President Reagan's admission of support for anti-Sandinista "freedom fighters" constitutes "a declaration of war" and a violation of international law.

U.S. Ambassador Jeane J. Kirkpatrick, equally alert to the national debate over Central America policy, replied that "there is no American invasion of Nicaragua. The problem for Nicaragua is Nicaragua. In Nicaragua, Nicaraguans are fighting Nicaraguans."

Nicaragua's Foreign Minister Miguel D'Escoto said that Reagan's "confessions" at his press conference last Wednesday gave the Sandinista government an opening to bring its case to the Security Council for the second time in six weeks.

The last debate ended without a resolution or a vote. In it, the speeches of the vast majority of delegates, including many American allies, endorsed the basic Nicaraguan position.

Nicaragua appeared to be pressing for a vote in the current round, however, circulating a mild resolution designed to attract maximum support. It refers to "aggressions and threats" by "a great power," rather than naming the United States. And it demands an end to overt and covert intervention against Nicaragua, but not against El Salvador.

It calls for a dialogue between Nicaragua and Honduras, and another between Nicaragua and the United States.

Finally, in a bow to pressure for regionwide accommodation, the resolution commends the peace efforts of the "Contadora group"--Mexico, Venezuela, Panama and Colombia. And it calls on U.N. Secretary General Javier Perez de Cuellar to use his good offices, in coordination wtih those four countries, "with a view to achieving a solution to the problems that affect peace in Central America."

The Contadora group is scheduled to meet again in Panama on Wednesday.

One Latin American diplomat said today that "Reagan put the ball there, and they the Nicaraguans swung. They'll get a majority vote for their resolution, an American veto, and in addition they'll stop the Organization of American States from acting." The OAS, based in Washington, is considered a less favorable diplomatic forum for Nicaragua.

U.S. officials, taking a conciliatory line with other Security Council members, said they too back the Contadora "process" and would be "receptive to any sensible solution so long as it tackles the whole Central American problem."

Kirkpatrick, in her statement to the council, said that Nicaragua's "insistence on bilateral rather than multilateral talks underlines its desire to resolve its internal problems while avoiding the issue of its export of revolution, war and misery to its neighbors."

She concluded that while destabilizing El Salvador, Nicaragua demands U.S. protection with the claim that its own people, "repressed by the foreign arms of a superpower the Soviet Union , have no right to receive help against that repression."

Both D'Escoto and Kirkpatrick showed sensitivity in their statements to the political debate over Central America inside the United States. The Sandinista foreign minister said more than 500 Nicaraguans have died in rebel attacks since the start of this year, and that 17 incursions from Honduras last month--including one by 1,200 insurgents on April 30--were repulsed.

The rest of his case was built on quotations from American congressmen, news stories and editorials. It concluded that the American people reject Reagan's policy.

Kirkpatrick replied that there is "indeed a debate in the U.S. about whether the U.S. should help the people of Nicaragua and El Salvador fight . . . the ruthless terrorist international. We will continue that debate and in the end we'll make our decision by democratic means. We wish there could be such a free debate in Nicaragua."

After her statement she stalked out of the council chamber, telling reporters that D'Escoto's words were "a tissue of lies."

When told of this remark he responded by saying, "she should wash her mouth out." He added that Kirkpatrick showed "unbelievable gall" by saying the United States has a right to use military pressure to influence Nicaragua's internal affairs.

D'Escoto made these remarks during a half hour of frequent pauses along a U.N. corridor to answer the questions from a crowd of American broadcast and print journalists.