President Reagan last night sought to reassure those who fear his administration is heading for a Vietnam-type war in Central America by discounting the importance of planned large-scale U.S. military exercises in the region and emphasizing that he favors peaceful resolution of armed conflicts in El Salvador and Nicaragua.

At a nationally televised news conference in the East Room of the White House, the 19th of his presidency, Reagan suggested that the concern about his Central American policy reflects an inaccurate assessment of administration aims by Congress and the media.

"First of all, there is no comparison with Vietnam and there's not going to be anything of that kind in this," Reagan said. "And maybe the people are disturbed because of the confused pattern that has been presented to them and the constant drumbeat with regard to the . . . suspicion that somehow there is an ulterior purpose in this."

In attempting to shift the emphasis, Reagan endorsed efforts by some Latin American countries to obtain a regional peace settlement and even offered a rare compliment to the leftist governments of Cuba and Nicaragua for recent statements "that seem to indicate that they, too, now recognize the merit to regional negotiations."

This contrasted sharply to recent Reagan statements in which he denounced what he called the Cuban and Soviet "war machine" in Nicaragua. But, while the tone of Reagan's comments last night was subdued, he did not back away from any of the Central America policy positions that have aroused opposition here.

Though he refused to discuss covert U.S. support for guerrillas challenging the leftist Sandinista government in Nicaragua, Reagan said it would be "a very grave mistake" for Congress to outlaw the CIA operation. Administration sources have put the total of the U.S.-backed anti-Sandinista guerrillas at about 12,000 men.

The president said no plan has been put before him to raise the number of U.S. military advisers in El Salvador from the current self-imposed limit of 55, although other administration officials have said the Pentagon has sent such a recommendation to the White House. But Reagan did say it would be possible to train Salvadoran troops more quickly for their war against leftist rebels if the number of U.S. advisers were increased.

Reagan repeated a statement from his last news conference, this time attributing it to President Franklin D. Roosevelt, that a president "should never say 'never' " when asked a hypothetical question about the commitment of U.S. troops.

He also said, as he has in the past, that he has no plans to commit combat troops to the region and that U.S. allies have not requested them.

Reagan's news conference last night, his second in a month but the last one planned before Labor Day, was dominated by the issue of Central America, as White House officials had anticipated. One said the president scheduled the news conference because he wanted to "calm the fears in the country and Congress" about the direction of U.S. policy in the region.

The president opened with a statement in which he emphasized his support of "democracy, reform and human freedom" in Central America and in which he played down the role of U.S. military and naval exercises that will begin next month in Honduras and off the coasts of Central America.

The exercises will involve two battleship groups and from 3,000 to 4,000 combat troops, but Reagan said they are "limited in purpose." He added that the United States has held "much larger scale exercises" in Europe, Asia and Latin America, although the Pentagon has said there have not been exercises of this magnitude in Central America. Despite U.S. opposition to "the use of force by one neighbor against another," Reagan said in a reference to Nicaraguan support of the leftist guerrillas in El Salvador, "we are not seeking a larger presence in that region, and U.S. forces have not been requested there. The United States stands firmly on the side of peace."

Shortly before the news conference began the White House released the text of a letter which Richard B. Stone, the special U.S. ambassador in the region, had delivered to the presidents of the four "Contadora" countries--Colombia, Mexico, Panama and Venezuela--praising their efforts to reach a peaceful regional settlement.

The letter restated previous policy declarations of the Reagan administration, outlining what the president believes to be the necessary conditions for a peaceful settlement to the guerrilla wars in El Salvador and Nicaragua.

Reagan said he anticipated no provocative incidents in either the military or the naval maneuvers that will begin next month unless it was a terrorist attack that could happen in a base in America. He said the military exercises in U.S.-supported Honduras "are not going to put Americans in any reasonable proximity to the border" with Nicaragua.

Reagan said that instructions to U.S. forces to return fire if they are fired upon are routine, adding that soldiers who "have taken it upon themselves to be our defenders and protectors have a right to defend and protect their own lives."

Saying the many U.S. warships, including aircraft carriers, included in the military exercises are not being sent near Nicaragua for any hostile action, Reagan added that a Soviet freighter, the Alexsandr Ulyanov, was currently approaching the Nicarguan port of Corinto carrying a load of transport helicopters for military purposes.

"And no one shot at them," the president added.

Reagan, in command of the questioning during most of the news conference, was asked near the end who was to blame for the "confused pattern" he said had appeared in recent media and congressional descriptions of his policy. "I don't think there is as much confusion as they are trying to point out about this," he replied.

Repeatedly, no matter what he was asked, the president stuck to his essential theme that his policies were promoting peace, not war, in Central America.

Asked whether the American people were ready to support a war to defend U.S. interests in the region, Reagan launched into a long answer in which he praised Americans as "the most peace-loving people in the world" and then reiterated what he said was his own longstanding opposition to war.

The reporter tried again, asking whether the American people supported Reagan policies in Central America.

"Those that have been informed and understand it do," Reagan responded. "I just met with some today who made it evident that they did. Now, if we all get together and explain what's happening down there, perhaps that'll resolve the situation in that regard."