Scientific tests of air, water and milk have shown that the accident at the Three Mile Island nuclear power plant near Harrisburg, Pa., poses no hazard so far to Washington-area residents, government officials said yesterday.

"We have very good evidence that there isn't any [hazard]", said Robert E. Corcoran, radiation control chief for the Maryland Health and Mental Hygiene Department. "We've seen no radioactivity above the natural background level."

Since Wednesday, when the accident occurred at the nuclear plant about 100 miles north of Washington, Maryland officials have tested samples of air, milk and water gathered mainly from northen parts of the state. Corcoran said none of the samples contained more than normal radioactivity. The water samples were taken from the Susquehanna River, which flows into the Chesapeake bay.

District of Columbia officials also tested air samples and reported no increase in radioactivity. "There has been no change in the radiological content in the air-there has been no change at all," said Herbert T. Wood, occupational and institutional hygiene chief in the D.C. Environmental Health Administration. "That is consistent with the direction of the wind, which was to the north."

The tests, which are expected to continue during the weekend and next week, were described by officials as precautionary measures prompted by the accident at the Three Mile Island plant, which had emitted radioactive gases into the air and the Susquehanna River yesterday.

Officials said the gases appeared unlikely to contaminate the water, fish or sheel fish in the lower Susquehanna River or in the Chesapeake Bay because the gases were expected to have dissiapted within a few miles of the nuclear plant site in Pennsylvania.

The officials said that the radioactive gases were not likely to be transmitted to the Washington area by air currents because the wind near Harrisburg was blowing toward the northeast, not toward Washington, and because the gases were apparently dissipating several miles after they had been vented.

Clifford Goodall, the meteorologist in charge of the National Weather Service's Harrisburg office, said that the wind was forecast to remain southwesterly through the weekend.

Some nuclear protest groups here expressed skepticism about government reassurances that no hazard appeared to exist in the Washington area.

"I am just absolutely outraged," said Fred Millar, a member of the Potomac Alliance, a Washington based antinuclear group. He contended that prospects of a major meltdown within the Three Mile Island reactor could pose severe health dangers for Washington area residents if radioactive substances were carried here by air currents.

The District of Columbia's key measurement yesterday of radioactivity in the air found 0.5 picacuries per cubic meter, according to Wood. He described this as about the same amount as is measured on a normal day. A picacury is one-trillionth of a curie, a basic radiation measure. Wood said the highest such reading ever recorded here was 1.7 picacuries per cubic meter, monitored after a Chinese atomic test in 1977.

By comparison, Wood said, officals would only urge residents to stay indoors to avoid radiation hazards if a reading reached 15 picacuries per cubic meter-about 30 times yesterday's measurement.

Despite the Three Mile Island accident, nuclear power plant officals in Maryland andVirginia reiterated their assurances yesterday that their plants are safe.

The nuclear generating stations in Maryland and Virginia include the Baltimore Gas & Electric Co.'s Calvert Cliffs plant, about 50 miles from Washington on the Chesapeake Bay, and three plants fun by Virginia Electric and Power Co.-North Anna Unit I, about 80 miles from Washington in Louisa County, and Surry Units I and II, about 40 miles from Washington in Surry County.

Robert M. Douglass, quality assurance manager for BG&E, said even a "worst case" disaster would be confined to about a 5-mile radius of the Calvert Cliffs plant.