The Soviet Union, which apparently canceled a planned test flight of a new intercontinental-range missile on the night that a South Korean airliner was shot down, did hold such a test three days later, according to administration sources.

The sources said that the test was of the new SSX24 missile and that it failed. This is believed to be the seventh failure in 10 test flights of the new three-stage, solid-fueled missile since it was first tested last October, the sources said.

The expectation by U.S. intelligence that the Soviets were about to make a missile test on the night of Aug. 31 (Washington time) was the reason, officials said, that an American RC135 reconnaissance plane was patrolling off the coast of the Soviet Kamchatka Peninsula in international airspace.

The peninsula, in the far northern Pacific, is the area where missiles land after being fired from test centers such as the one at Plesetsk, several thousand miles away in the central Soviet Union.

The reconnaissance planes, military versions of the four-engine 707 jetliner, and U.S. intelligence ships with radar aboard, are used to try to pick up electronic signals from the test missiles as a way to measure Soviet compliance with various arms control agreements.

At one point during the night of Aug. 31, the RC135 and Korean Air Lines Flight 007 passed within 75 miles of each other while both were in international airspace. The Korean jetliner later strayed inexplicably off course hundreds of miles into Soviet airspace and was shot down some 2 1/2 hours later by Soviet fighters. The reconnaissance plane, according to the administration, was by then 1,000 miles away and back at its base in Alaska.

The Soviets, according to both American and Soviet accounts, initially thought that the plane in their airspace was a U.S. reconnaissance plane. The Americans contend that there should have been no conceivable way, however, for the Soviets to have mistaken the distinctive 747 Korean jumbo jet for the smaller RC135 once Soviet fighter pilots tried to intercept it.

According to American officials, the RC135 returned to its base because of indications that the planned missile test had been canceled. Officials said that they did not know for sure if the test that had been planned for the night of Aug. 31 was of the same missile that was test fired three days later. The Soviets have been testing two missiles in recent months that are of particular concern to the United States. Both use solid fuel and both have experienced failures.

The Soviets have identified the SSX24 as a new medium-sized missile roughly the size of the U.S. MX missile now being developed. Under previous arms control agreements, each side is allowed one new missile.

But the Soviets have also made four tests thus far of the PL5 missile, a smaller and probably mobile solid-fuel weapon. The United States claims this missile is also a new one and thus a violations of previous accords.