The Soviet Union and eight other nations signed an agreement Friday setting the ground rules for the mineral exploration of Antarctica, which scientists believe holds vast riches beneath its icy surface.

The United States and three other nations were expected to sign the accord in the near future.The agreement, which climaxed six years of negotiation by 38 countries of the Antarctic Treaty Organization, means Antarctica soon will no longer be "the virgin continent," although speakers at the signing ceremony said the accord was vital to ensure the ecological stability of the 5.5 million-square-mile wilderness.

Untold billions of dollars worth of mineral wealth are reported trapped beneath Antarctic's frozen terrain and until recent times it remained unaccessible because of the absence of appropriate technology to mine it.

A Brazilian report said there was evidence of enormous reserves of coal, iron ore, uranium, silver, petroleum and carbon. The coal seam alone was 25 miles wide and 60 miles long, the report said.

Along with the Soviet Union, other nations signing were New Zealand, Norway, Finland, Brazil, South Korea, South Africa, Sweden and Uruguay.

The United States, Britain, Chile and Argentina said they would sign the agreement as soon as possible.

The U.S. Ambassador to New Zealand, Paul M. Cleveland, said the signing was "an historic occasion in international co-operation in the management of Antarctica." He said he would sign the convention for the United States "within the next few days."

Antarctica has never been mined. The member nations of the Antarctic Treaty Organization had agreed voluntarily not to exploit it until the written agreement was made.

After the ceremony, New Zealand's Foreign Minister Russell Marshall said the accord recognized that an unregulated scramble for resources on the ice continent could produce major political and legal conflicts.

"The consequences would be a loss of stability in Antarctica, the end of the tradition of friendly and open cooperation . . . and almost certainly severe damage to the Antarctic environment," Marshall said.

"This convention does not open up Antarctica for mining," Marshall said. "It does not signal the death knell for the Antarctic environment."