The Soviet Union has exploded six SS-12 missiles that were removed from Eastern Europe under a superpower treaty to eliminate all land-based intermediate-range nuclear weapons, Radio Moscow said Monday.

The missiles were exploded at a test site in southern Kazakhstan, a republic of Soviet Asia where they had been brought by train from East Germany and Czechoslovakia on Feb. 25-March 13. Nuclear material was removed before the missiles were exploded, the official radio said.President Reagan and Mikhail S. Gorbachev signed an agreement in Washington on Dec. 8 to eliminate the medium- and shorter-range missiles, which have a range of 300 to 3,100 miles. The SS-12s have a range of 560 miles.

The treaty is awaiting ratification by the U.S. Senate and the Presidium of the Supreme Soviet, the Soviet Union's nominal parliament. Passage by both sides is considered a virtual certainty.

"The first six Soviet shorter-range missiles liable for scrapping under the INF treaty with the United States have been exploded at a test site in southern Kazakhstan," Radio Moscow said in an English-language broadcast.

The explosions took place near the city of Saryozek, 125 miles north of the Kazakhstan capital of Alma Ata. Radio Moscow did not say when the missiles were destroyed and gave no indication when more missiles might be exploded.

It said that under the terms of the treaty, the missiles, launchers and all auxiliary equipment are to be destroyed.

Krasnaya Zvezda, the Defense Ministry newspaper, said in an article published on Sunday that local residents were worried about possible environmental damage but that there was none.

The paper said an area of two miles around the missiles was considered a danger zone and that local people were warned far ahead of the blast.