MOSCOW, DEC. 15 -- South Korean President Roh Tae Woo said today that Soviet leader Mikhail Gorbachev has promised to do everything within his power to promote the reunification of the two Koreas.

Addressing a press conference at the end of the first official visit to Moscow by a South Korean leader, Roh said he and Gorbachev had agreed to a dramatic expansion in economic cooperation.

Soviet officials predicted that trade between the two countries, which stood at less than $600 million in 1989, could soar to more than $10 billion a year by the mid-1990s.

Scarcely imaginable only a few years ago, the blossoming friendship between Moscow and Seoul after four decades of hostility is a dramatic illustration of the casting aside of ideological ambitions by the Kremlin in pursuit of economic advantage. It also increases the international isolation of the North Korean regime of Kim Il Sung, who has relied on the Soviet Union and China for arms and political support since the 1950-53 Korean War.

Roh's visit to the Soviet Union came just three months after the two countries established diplomatic relations, putting an official stamp on their growing economic ties.

Kremlin officials clearly are hoping that economic agreements signed this week will pave the way for massive Korean investment in the Soviet Union's crisis-ridden economy, providing a rapid injection of consumer goods and technological expertise.

For Seoul, the Soviet Union represents a valuable source of raw materials. South Korean officials also hope their new relationship with Moscow will help them achieve a diplomatic breakthrough with the North.

Roh quoted Gorbachev as saying that it was necessary "to be patient and carry on dialogue between North and South and create an atmosphere of mutual trust," which would create suitable conditions for reunification.

Soviet Foreign Ministry spokesman Vitaly Churkin said the Soviet Union wants to "develop relations" with Seoul while maintaining its present ties with Pyongyang. The North Koreans reacted angrily to the ties between Moscow and Seoul, but have since toned down their criticism, a signal that they regard the new relationship as a fait accompli.

The Soviet news agency Tass quoted Gorbachev as linking the reunification process to creation of a nuclear-free zone on the Korean peninsula. The Kremlin has called for the removal of U.S. nuclear weapons from South Korea, arguing that they are an anachronism in the new international climate.