Ambassador-at-large Vernon Walters arrived here today to put added U.S. pressure privately but firmly behind the efforts of three battling political parties to form a government of national unity.

Walters, a retired general and former deputy director of the Central Intelligence Agency, came on what the U.S. Embassy, refusing comment on his mission, called a private visit. Walters refused to be interviewed, but his arrival is a clear measure of the anxiety with which the State Department is waiting for Salvadoran politicians to resolve their bitter differences and get down to work.

U.S. officials in Washington said Walters is in El Salvador to get a "status report" on the political situation there. Walters traveled to San Salvador from Buenos Aires where he had accompanied Secretary of State Alexander M. Haig Jr. on his mission to find a solution to the Falklands Islands crisis.

According to Salvadoran political leaders, the embassy sent them a letter outlining U.S. policy goals here and inviting them to a meeting with Walters late today.

"It sounded like it would be a lecture," said one party official, a newly elected member of the Constituent Assembly, which held its first meeting yesterday.

He said the letter reminded the parties that continued U.S. aid was contingent on their forming a government acceptable to Congress, one in which all three major parties are fully represented.

The letter also said the United States wants to see a government that respects human rights and retains the land reform and other social programs of the ruling Christian Democrats, said the official, adding: "We know that already."

But in separate interviews, several politicians said it may be a long time before the parties reach agreement. "We can talk a long time and still say nothing," one remarked.

Two rightist parties which together will dominate the assembly elected March 28 are seeking to minimize the participation of the Christian Democrats, who took 35 percent of the votes and 24 of the 60 assembly seats. The Christian Democrats are holding out for a government headed by a three-member civilian junta, with one member from each of the three main parties, and a major share of Cabinet posts.

John Carbaugh, an aide to Sen. Jesse Helms (R-N.C.), also arrived in San Salvador today, apparently also on a private mission related to the new government. He declined to specify what it was.

In another development, free-lance photographer John Hoagland was arrested by the Salvadoran Treasury Police this afternoon in the parking lot of his hotel on swindling charges he said were related to a dispute over a van he rented last year. The van was wrecked in Honduras.

Hoagland was taken to the Treasury Police headquarters and held while U.S. Embassy officials worked for his release.