Steel imports jumped nearly 12 percent in April from the previous month, adding to the problems of U.S. steelmarkers already battered by a sharp drop in orders because of the recession.

The American Iron and Steel Institute said today 1.25 million tons of foreign steel entered the United States last month, compared with 1.07 million tons in April 1979 and 1.12 million tons in March.

So far this year, foreign mills have shipped 5.3 million tons to the U.S. market. During the same period in 1979, imports totaled 4.76 million tons, the Washington-based trade association said.

"At a time when every day brings news of layoffs in domestic mills and when our raw steel production is the lowest in almost five years, the increase in imports is particularly damaging," an AISI spokesman said.

Thousands of steelworkers have been laid off in recent weeks as bookings for steel products plunged, forcing the mills to cut back on production.

For instance, the nation's largest producer, U.S. Steel Corp., has announced it will indefinitely shut down its Edgar Thomson Works outside of Pittsburgh this weekend because of lack of demand for steel. About 2,000 workers will be affected.U.S. Steel also will lay off 500 more workers at its Fairless Works next week, a spokesman said.

And Bethlehem Steel Corp. is reviewing possible production cutbacks at its Johnstown works, a spokesman said today. The company said declining orders may force it to take one of three open-hearth furnaces out of operation sometime in June. About 750 of the plant's 7,700 employes would be affected. s

Bethlehem, which already has made layoffs at its Sparrows Point, Md., Burns Harbor, Ind., and Lackawanna, N.Y., facilities, also said it will close its Johnstown winter plant in August, as it prepares for start-up of an electric furnace shop in 1981. About 110 employes will be effected by that move.

At Armco Inc., a spokesman said 190 more workers will go on layoff status at its Middletown, Ohio, works, effective Sunday, bringing total cuts there to 511.

An association representing about 670 leading importers says the April figures reflect the brisk demand for steel at the end of last year.

"April imports are based on orders principally placed in December, 1979, when the market demand was still relatively brisk," said Kurt Orban, president of the American Institute for Imported Steel.

"These orders came before the sharp decrease in American steel demand in March and before the dumping complaints against overseas producers," he said.

The spokesman for the domestic steel group conceded that import figures reflect earlier bookings. "But regardless of the reasons, the effect is that imports tend to make worse an already difficult market situation," he said.

About 20 percent of the imported steel comes in the form of cold-rolled and hot-rolled sheets, which are used primarily in the auto industry.

But because the U.S. automakers are using about one-third less steel than they did a year ago, the importing of sheet goods now is "very, very disruptive," the AISI spokesman said.

The slowdown in steel was underscored by the AISI's report earlier this week that production for the week ended May 24 plunged to 2 million tons -- the second-lowest weekly figure since 1975, an off-year for the industry.

To date, domestic mills have produced 50.5 million tons, about 10.3 percent less than the 56.3 million tons poured last year for the same period, AISI said.