The United States has told Libya, through the Soviet Union, it is deeply concerned about reports Libya has been trying to obtain chemical weapons from Iran in exchange for sophisticated Soviet-made mines, a senior administration source said yesterday.

The official implied that the Soviets and the Americans, in a rare example of U.S.-Soviet cooperation, had jointly pressed Libyan leader Moammar Gadhafi to not swap weapons with Iran.

Gadhafi later complained in a letter to U.N. Secretary General Javier Perez de Cuellar that Washington had "threatened" him. The administration source said the United States had talked to the Soviets about the reports of a swap of mines for chemical weapons and expressed "the seriousness with which we've view such a development."

"We take very seriously reports Iran is seeking sophisticated mines from Libya," the official said. "We do know Iran has been moving around to various sources trying to improve its {mining} capability."

It was unclear whether the United States believes Libya has provided Tehran with some Soviet-made mines. The Libyans, in their letter to Perez de Cuellar, denied that they had.

Libya and Syria have been supporting Iran and providing it with some Soviet-made weapons, including long-range surface-to-surface missiles.

Yesterday, Libya appeared to be signaling its decision to change sides in the Iran-Iraq war, by restoring diplomatic relations with Iraq and calling for a halt to hostilities.

Recently, Chad complained that Libyan forces fighting in northern Chad had used chemical weapons against its troops.

The senior U.S. official noted Chad's complaint but said that "to our knowledge" Libya does not have chemical warfare capability.

By contrast, the official said the United States took seriously reports that Iran was seeking to obtain more sophisticated mines than it has so far used to disrupt shipping in the Persian Gulf.

The Soviet Union and the United States have been separately stepping up pressure on Tehran to accept a U.N. Security Council resolution, adopted in mid-July, demanding a cease-fire in the seven-year-old gulf war.

This joint approach through the United Nations to end the war has apparently provided the basis for the U.S.-Soviet effort to stop Libya from either obtaining chemical weapons from Tehran or providing Iran with sophisticated Soviet-made mines.