The Reagan administration, which made El Salvador the symbol of its determination to block communist interference in the Third World, changed direction yesterday and asserted that the American press is focusing too much attention on U.S. aid to that Central American country.

"Our impression is that this story is running about five times as big as it really is," a senior State Department official told reporters at a background briefing. The official, who cannot be identified under the rules of the briefing, added that the administration would like reporters to write less about El Salvador and more about other issues.

The official -- one of the principal spokesmen for the administration in warning of the dangers of communist involvement in El Salvador -- stuck to that assertion even after conceding that the administration, with Secretary of State Alexander M. Haig Jr. in the lead, had orchestrated a publicity-fraught crisis atmosphere with its charges of massive arms smuggling by Cuba and other communist countries to El Salvador's leftist insurgents.

Initially, the official admitted, the administration, seeking to attract a worldwide audience for its charges, had encouraged attention through such tactics as sending high-level teams of officials to confer with other governments, calling in congressional leaders for special briefings and peppering Cuba with rhetorical warning shots such as Haig's threat to "go to the source" in stopping the arms flow.

But, he also admitted, once the administration got its message delivered, it had hoped the press would turn its attention elsewhere. Although he wouldn't say so directly, the official left the impression that the administration is not happy with the recent publicity given to the dispatch of more U.S. military advisers and arms to El Salvador -- especially since much of the coverage has focused on comparisons to Vietnam and charges that the United States is backing an unpopular government aligned with rightist forces.

Another senior department official privately was more forthcoming. He said: "The administration wanted to focus attention on certain parts of the problem. But once they did that, they found that they couldn't keep it selective -- that there was no way to control it and keep it from growing as a subject of press interest."

He and other administration sources said that yesterday's aboutface was the result of three major considerations.

One, they said, is the desire of President Reagan's inner circle of White House advisers to direct the main thrust of press attention toward the president's economic program, and their growing unhappiness over the way the El Salvador issue has deflected their efforts.

Another factor, they said, involved concern within the State Department that the administration is being perceived at home and abroad as so obsessed with El Salvador that it is not giving sufficient attention to other major foreign policy issues such as the Middle East, Poland and Afghanistan.

Finally, the sources said, leaders of important allied governments, who have conferred recently with Reagan and Haig, cautioned that maintaining a highly visible, tough stance was having counterproductive effects in their countries and could hinder their support of U.S. policy. That message is understood to have been given Reagan in especially forceful terms by Canadian Prime Minister Pierre Trudeau during their meeting in Ottawa earlier this week.

At yesterday's briefing, the official said the lull in the arms flow through neighboring Nicaragua continues, and he added there is evidence that could indicate a slowing of arms shipments from Cuba to Nicaragua. But, while saying "we've moved into a period that looks more favorable," he cautioned: "I'm not willing to call it a victory yet."

He also said that because Nicaraguan authorities appear to be cooperating in efforts to halt the arms smuggling, the administration is easing the 30-day deadline set by Haig for a definite cutoff of U.S. aid. That deadline is about to expire, but the official said Nicaragua's efforts have created "a different ball game that puts us in a different time frame" in respect to immediate decisions about terminating aid.