The Navy could blockade El Salvador "against all comers" but not without cutting into its strength elsewhere, Secretary John Lehman said yesterday.

Stressing that he was making no recommendation either for or against setting up a blockade to stop arms going from Cuba to Nicaragua and then to El Salvador, Lehman acknowledged that such a blockade would strain the U.S. fleet.

"When you do it, you must be aware that with a 400 Navy-ship fleet, you're going to use a proportion of that" for the blockade, meaning that "your ability to carry out military tasks elsewhere is proportionately drawn down," Lehman told reporters at a breakfast session.

He dismissed a suggestion that blockading a little country such as Nicaragua would be relatively easy for the U.S. Navy, saying: "You don't go into a blockade unless you're prepared to see that it's not broken. That means you have to look at where are the potential blockade runners coming from and what kind of equipment are they going to use?

"If a Soviet Kirov battle cruiser is going to try to run the blockade," Lehman said, the U.S. Navy would have to have ships on hand capable of combating it.

"There is no such peacetime blockade," the Navy secretary added. "A blockade is an act of war under international law. All I would say is that before you engage in an act of war you make sure that you are prepared to deal with" the threat and that "the Congress fully supports, and the American people support, the actions you are going to engage in."

Senior Defense and State department officials have told reporters on a background basis--meaning not for attribution--that they believe the American public is a long way from supporting the commitment of U.S. forces in Central America, largely because of the never-again hangover from the Vietnam war.

Lehman said top administration officials agree that "never again will we get in any military action without the solid support of the government and people. We have the capability to prevail in a blockade against all comers," Lehman said in talking about strictly military capabilities.

However, his stress on the political inhibitions against taking such an action hardened the impression that the Reagan administration regards the blockade as a distant option.