President Reagan said yesterday that there is no "blackmail intended" in his telling Congress that it must approve $110 million in new military aid for El Salvador or the United States may have to send more military advisers into the civil war there to help train government forces.

In an 11-minute news conference with reporters in the White House briefing room the president dismissed criticism that his view of the communist threat to El Salvador is "overblown." He said fears that a large American role in El Salvador will lead to U.S. involvement approaching the scale of the Vietnam war are ill-founded because no combat troops will be sent to El Salvador.

"I do not see why there is so much opposition to it," Reagan said of his proposal for increased military aid to El Salvador. "We have no intention of sending combat forces, nor have we ever been asked for combat forces. And there is no intention of us sending the adviser teams to be with combat units or anything. We are talking about simply giving their military some of the fundamental training to enable them to do the job."

Reagan was asked whether the United States was making an open-ended commitment to the government in El Salvador, but he did not answer directly. He said only that no date could be given for the termination of U.S. involvement in the conflict.

When he was told that House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) had said that on El Salvador there is "a strong feeling the president has gone too far," Reagan replied:

"You know, with regard to the speaker, us Irish are given to oratory--sometimes flamboyant."

The president, who said Thursday that the United States has been "slow to understand" the threat of a communist takeover in the Caribbean and Latin America, said yesterday that the rebels are not "discontented peasants" but trained soldiers equipped by "outsiders by way of mainly Cuba."

"I would like to call attention also," Reagan said," that in our international aid, in many of the troubled spots of the world, it does seem peculiar that this is the only one where they Congress seem to be raising objections, and yet here is one that is a threat to the Western Hemisphere, to our own security, in fact."

The president added that the government of El Salvador is "making every effort to persuade the guerrillas, to offer amnesty, to persuade them to come in and participate in the democratic process and not try to shoot their way into a ruling position in government."

While El Salvador's government is considering an amnesty plan, it has not yet made an offer to the rebels.

At his news conference the president also:

* Said he does not know details of a report that the Soviet Union had expelled an American diplomat on charges of spying.

* Praised Congress for "bipartisanship" in its work on a legislative package to keep the Social Security system solvent and on a jobs creation bill. But the president added that he is "deeply disturbed" that the jobs bill may become "a Christmas tree for special-interest legislation. We must firmly oppose that."

Before the news conference, Labor Secretary Raymond J. Donovan announced that the administration is sending to Congress "the employment act of 1983."

While the jobs bill now in Congress is intended to alleviate the immediate unemployment problem, the new one reportedly attempts to deal with structural unemployment caused by the shift in the job market to high technology.