The United States is prepared to conduct another underground nuclear weapons test within the next few days, according to informed sources.

Whether the U.S. test shot, the second this year, will take place before or after Monday is unclear, they said.

If it comes after that day, it would trigger the end of the Soviets' self-imposed eight-month test moratorium, according to statements earlier this week by official Soviet news services in Moscow.

The first U.S. test took place last Saturday when Los Alamos National Laboratory scientists exploded a device as part of the development of a new warhead for the proposed Midgetman mobile, land-based intercontinental missile, according to sources inside and outside the government.

Rep. Thomas J. Downey (D-N.Y.), who organized a group of 60 legislators in a last-minute plea to President Reagan to halt last week's test, said yesterday that he had not heard about the new test but wondered "how long the administration can be out by itself testing nuclear weapons and ignoring world opinion."

The next test, also to be run by the Los Alamos group at the Nevada Test Site, was originally scheduled March 15, according to sources. The purpose of the shot has not been disclosed.

Energy Department officials normally do not comment on such tests and do not announce them in advance unless they are scheduled to involve at least 20 kilotons, equivalent to an explosion of 20,000 tons of TNT.

The forthcoming test was postponed so Reagan could make a testing offer in response to a statement March 13 by Soviet leader Mikhail Gorbachev that Moscow's test moratorium would continue after next Monday but only until the first U.S. test after that date.

The Reagan administration has refused to participate in Gorbachev's moratorium and has refused to resume negotiations for a comprehensive test-ban treaty that would bar all tests. Instead, Reagan has proposed that both nations seek better verification of the size of tests.

That would permit a move toward ratification of the 1974 threshold test-ban treaty limiting each nation's tests to explosions of 150 kilotons or less. The bomb that destroyed Hiroshima in August 1945 measured 12 1/2 kilotons.

U.S. officials have maintained that, as long as nuclear weapons are required to deter war, tests are needed to keep the U.S. stockpile modernized and reliable.

The approximately 16 U.S. underground tests planned this year involve weapons development; tests of warheads in production; experiments to elucidate the physics of nuclear explosions; so-called effects tests to see how radiation impacts on land-, sea- and air-based weapons, and infrequent checks of how long-stockpiled weapons work.

The United States, in its biggest nuclear weapons building program in 20 years, has begun or is about to begin production on a new warhead for the MX intercontinental missile; the Trident I sub-launched intercontinental ballistic missile; air-, sea-, and ground-launched cruise missiles; the B61 tactical bomb and B83 strategic bomb; the Pershing II missile, and 8-inch and 155-mm. artillery shells.

In development, along with the Midgetman warhead, are the Trident II sub-launched missile warhead, a new anti-submarine nuclear depth charge, a ship-based SM2 nuclear antiaircraft missile, and an aircraft-launched, short-range SRAMII nuclear missile.

Last Sunday, White House chief of staff Donald T. Regan defended U.S. testing, saying that "the Soviet Union had a very intense testing period over the last 12 months. They finished their tests. Then they said, okay now we're finished, no more."

In the 12 months preceding Gorbachev's moratorium, 16 Soviet nuclear tests were reported, according to the Natural Resources Defense Council, the same number reported a year earlier.