Secretary of State Alexander M. Haig Jr., continuing his tough talk about Latin America, yesterday characterized Cuba's alleged smuggling of arms to El Salvador as conduct "no longer acceptable in this hemisphere" and threatened "to deal with this matter at its source."

Pressed to say what action the United States might take, Haig answered, "If I ever learned anything, it's the sterility of standing in front of the American press and laying out what you intend to do in a contingency."

Haig also charged explicitly for the first time that Nicaragua's involvement in arming El Salvador's leftist insurgents "is in violation" of U.S. law governing aid to Nicaragua, and he warned that the United States may cut off resistance to the Managua government.

However, the secretary later softened his comments on Nicaragua to make clear that the administration is reserving a final decision on an aid cutoff until it has a clearer idea of whether Nicaraguan authorities are trying to halt the arms flow through their territory to neighboring El Salvador.

Haig's comments were the most bellicose he has made publicly in the two weeks since the question of communist support for the leftist guerrillas in El Salvador's bloody civil war became a matter of priority for the administration.

The remarks came during Haig's participation at a White House briefing on the administration's budget and after a subsequent meeting at the State Department with the British foreign secretary, Lord Carrington.

Asked at the White House about Cuba, Haig repeated in public the theme he had sounded in a private briefing last week for ambassadors from the allied countries. He had asserted then that the United States would not repeat its Vietnam war error of involving itself in a conflict without dealing with instigating forces on the outside.

Yesterday he said: "I think we have made it very clear from the outset that the problem is emanating first and foremost from Cuba.And it is our intention to deal with the matter at it's source."

Later, as he was saying goodbye to Lord Carrington, Haig added, "I think the situation is clearly a situation in which Cuban activity has reached a peak that is no longer acceptable in this hemisphere whether it be in El Salvador, Honduras, Guatemala or any other of our soverign republics. And it is our view that this is an externally managed and orchestrated interventionism, and we are going to deal with it at the source."

U.S. officials in recent days have reported a "lull" in the flow of outside arms to El Salvador. Haig said yesterday there was "conflicting evidence" on this point.

He also said, "There's evidence to suggest there are still vast amounts [of weapons] yet to be moved and a great deal of activity designed to move it. . . . All this is being very carefully watched."

Apparently mindful of the strong concern about political and social conditions in El Salvador expressed by Carrington and French Foreign Minister Jean Francois-Poncet here this week, Haig said the United States is not necessarily comfortable with the current level of social reform there and is urging its extension.

Responding to questions about the activities of Nicaragua's leftist-dominated government, Haig referred to conditions attached by Congress last year when it voted a $75 million aid package for that country. A balance of $15 million from that package and another $15 million in Food for Peace assistance is now suspended because of the arms flows through Nicaragua.

Haig noted that "the law provides that we cannot provide such assistance to governments actively involved in the export of terrorism and the support of agression abroad. And that is an issue which we know today that the government of Nicaragua is in violation of . . . .

"We're making appropriate adjustments that might be necessary should such a decision to terminate aid to Nicaragua be called for."

Later though, he toned down his remarks by switching tenses and saying Nicaragua "has been" violating the congressional provision. Asked whether the United States is losing patience with the Nicaraguans, he replied:

"We are not proceeding with public tests of manhood or deadlines but rather a very careful assessment of what remedial steps the government of Nicaragua is taking and has taken -- they have taken some -- we are continuing to watch it, and I will leave it there."

A further clarification was offered by State Department spokesman William Dyess who said Haig's references to Nicaraguan conduct involved "what has taken place in the last five or six months." Dyess added that Haig "is leaving open the question of what may happen now."

The National Security Council met yesterday, reportedly to discuss the Salvadoran situation, but reliable sources said no decisions were expected about actions against Cuba or levels of expected U.S. increases in military and economic aid to the Salvadorn government.

Haig also was asked his views about Robert White, whom he fired as ambassador to El Salvador and who tolda congressional subcommitee Wednesday that the principal threat in that country comes not from the left but from the right.

Haig replied: "I think any individual has every right to express his views, and we have every right to asses those views as we have and to take appropriate action with respect to them which we have."