Reagan uneasy over El Salvador?

President Reagan believes the Salvadorean government could lose the war against leftist rebels, according to the Washington Post, quoting unidentified administration sources. And he will take all ''necessary measures'' to ensure the government's victory, the sources added.

The reasons for the President's assessment, the newspaper said, were the rebels' battlefield successes, the declining morale of the Salvadorean military, and the reluctance of Congress to send more military aid money and American advisers. But other administration officials traveling with the President told United Press International they were surprised by the report and could not explain what was meant by ''all necessary measures.''

Earlier, the US threw its weight behind early elections in El Salvador. The Salvadorean government had promised presidential elections no later than March 1984. But US officials now expect El Salvador to accelerate that schedule and to offer a general amnesty to the guerrillas.

Meanwhile in San Salvador, a US citizen jailed on charges of belonging to a guerrilla organization was freed and all charges were dropped, a US Embassy spokesman said. The Defense Ministry's press office identified the man as Jorge Enrique Zapporoli, and said he was with four Salvadoreans in a car carrying handguns and shortwave radios.