The Soviet Union and the United States today opened an improved, more reliable "hotline" that uses satellites instead of telephone wires and undersea cables to link the White House and the Kremlin.

The direct teletype link, in operation since 1963, is meant to be used during periods of increased international tension or emergency. "In reality, the hotline is a direct private communications link between the president of the U.S. and the president of the Soviet Union," a U.S. embassy statement asserted today.

It did not say, however, whether President Carter or Soviet President Leonid I. Brezhnev have ever used the system to exchange messages, or if so, how many times. An embassy spokesman said he had "no way of knowing" the answer to such questions.

Until today, the hotline has been subject to unexpected interruptions -- potentially a grave problem in event of tension between the two governments, which both have the ability to launch theremonuclear weapons at each other at any moment.

The embassy said today that the hotline has been cut at least three times since 1963 including an instance when a Finnish farmer cut a buried cable while cultivating his land.

The new dual system using the synchronous communications satellites, Intelsat, and the Soviet Molniya system of satellites, should be virtually foolproof, the Americans said.

Tass, the official Soviet news agency, today quoted Vasily Shamshin, first deputy minister of communications, as saying that the new satellite link "will serve to strengthen trust and understanding between the two countries."

The hotline was organized by the two nations after the Cuban missile crisis in the fall of 1962, when delays in communications between the two countries added unnecessary confusion and apprehension. The original Washington-Moscow hotline was put into service Aug. 30, 1963.

The present improvements were called for as part of the 1971 strategic arms limitation talks. The dual systems use separate ground stations to beam and receive messages to and from the satellites. There is a station serving the Molniya system in the U.S. and two Intelsat earth stations on Soviet soil, one in Moscow, the other in Lvov, a city in the western Ukraine.

The embassy said that the U.S. link was "essentially complete" by the end of 1974, but that the Soviet earth stations went on for another two years to perfect it. "It appears that both systems now meet the technical standards called for under existing agreements, the embassy statement said.