Viktor Komplektov, an experienced specialist on American affairs, is Moscow's choice as the new Soviet ambassador here, administration officials say.

His appointment, due to be made public soon, puts a diplomat with a hard-line reputation in the key post as U.S.-Soviet relations are tested in the Middle East and the Baltics.At 59, he is moving into a job vacated by Alexander A. Bessmertnykh, who advanced to foreign minister after Eduard A. Shevardnadze resigned late last year with a blast at conservative trends in the Soviet government.

The administration officials who disclosed the selection of Komplektov on Monday did so on condition they not be identified by name. They said the United States had accepted the appointment of Komplektov.

"He is generally viewed as somewhat humorless and as a hard-liner, even occasionally polemical," a U.S. official said.

The United States is looking for Soviet support in the drive for Arab-Israeli negotiations and for continued world pressure on Iraqi President Saddam Hussein.

In the Baltics, the Bush administration is looking for concessions to the independence movement, hoping Lithuania, Latvia and Estonia go their separate ways.

Komplektov, 59, has held various posts in the Soviet Embassy in Washington. In Moscow, he was director of the U.S. department in the foreign ministry from 1978 to 1982, when he moved into his current job, deputy minister for foreign affairs.

Despite his long background in U.S. affairs, Komplektov was shifted to deal with Latin America while Shevardnadze, who is regarded as a liberal, was foreign minister.

Secretary of State James A. Baker III said at a news conference Friday in Moscow that U.S.-Soviet relations had passed through a difficult period. He credited Gorbachev with attempting to arrange a dialogue with leaders in the independence-minded Baltic republics and with trying to shift the sagging Soviet economy toward a Western-style market system.