President Reagan believes the civil war in El Salvador is approaching a critical stage and he will take "all necessary measures" to ensure the victory of the U.S.-backed Salvadoran government, administration officials said here today.

These officials said Reagan has been informed and believes that the Salvadoran government could lose the war because of the increased battlefield successes of the leftist rebels, the declining morale of the Salvadoran military and the reluctance of the U.S. Congress to pour more military aid money and American advisers into the Central American country.

The officials said that intense negotiations with Congress about greatly increasing U.S. assistance to the El Salvador military, based on the Reagan administration's newly pessimistic assessment of the situation, also are at a critical stage.

In addition to seeking from Congress an additional $60 million in U.S. military aid to El Salvador for this year, the administration is considering allowing U.S. military advisers to operate closer to Salvadoran troops in the field, according to government officials in Washington. Details on Page A21.

Reagan was said by officials here to be hopeful that a promise by the government in El Salvador to speed up its timetable for democratic elections, which has been negotiated by U.S. and Salvadoran officials, would help convince Congress to provide the increased military aid. One administration official said the Salvadoran government is planning other changes, which were not specified, that he said could help influence Congress.

"Unless we get the Congress in gear, we lose the war," said one official. "I'm not a great student of dominoes, but if we lose El Salvador we lose the region."

This official said Reagan is determined to prevent "a Marxist takeover" in El Salvador and has chaired three meetings within the past 10 days of an inter-agency group reviewing the Central American situation.

As evidence of this determination, the official pointed to Reagan's remarks in his speech to an American Legion meeting in Washington last week when he said: "We face a special threat in Central America where our own national security is at risk. Central America is too close to us, and our strategic stake in the Caribbean sea lanes and the Panama Canal is too great for us to ignore reality. The specter of Marxist-Leninist controlled governments in Central America with ideological and political loyalities to Cuba and the Soviet Union poses a direct challenge to which we must respond."

While officials did not specifically rule out the use of U.S. combat troops in Central America, they said the president was not considering any proposal to do so. They said Reagan instead wants a sharp increase in U.S. military aid to demonstrate to El Salvador and other nations in the region that the United States does not intend to abandon its friends in Latin America.

Some Salvadoran military units were described by administration officials as nearly demoralized, with declining ammunition stocks as low as 30 days' supply.

Administration sources said that Reagan plans to address the critical situation in El Salvador within a week in a public statement or prepared response to questions anticipated from reporters.

Administration sources said increased U.S. concern over the military situation in El Salvador reflected several factors.

One source cited a gloomy U.S. intelligence estimate of Salvadoran military capability in the face of mounting guerrilla pressure. Others said a key reason for the president's concern was the "frank and revealing" personal report he received from U.N. Ambassador Jeane J. Kirkpatrick on her return from a Latin American tour last month.

"She spoke with a note of gravity," said one official. "Though she didn't use these words, she said, in effect, 'If we don't get with it in El Salvador, we'd better write it off.' This the president is not prepared to do."

Another factor, said one official, was a Washington Post report that Thomas O. Enders, assistant secretary of state for inter-American affairs, had prepared a working paper envisioning negotiations with the Salvadoran rebels through a third party. Reagan was described as angry that that report might suggest a weakening of U.S. resolve. Subsequently, both Enders and Secretary of State George P. Shultz denied any consideration of negotiation with the rebels.

But administration officials acknowledged that the working paper did discuss the possibility of negotiations with leftist political exiles outside El Salvador who have links with the guerrilla movement.