WASHINGTON (AP) _ The Soviet Union is removing Deputy Foreign Minister Yuli Vorontsov from his job as chief negotiator with the United States on nuclear arms control and replacing him with a veteran deputy, Alexei Obukhov, a senior U.S. official said Friday.

This frees Vorontsov, who is one of General Secretary Mikhail S. Gorbachev's chief trouble-shooters, to concentrate on Afghanistan and other regional issues.

The Soviets apparently have decided to withdraw their forces from Afghanistan this year and are preparing for a new round of negotiations next month between the pro-Moscow government in Kabul and Pakistan, which backs the rebel resistance.

Obukhov is a seasoned negotiator who earned a master's degree at the University of Chicago and is fluent in English. The chief U.S. negotiator is Max M. Kampelman, but like Vorontsov he has spent little time in Geneva where the negotiations are held.

Secretary of State George P. Shultz has had Kampelman, who is also the State Department counselor, work on problems involving the Middle East and the war in Nicaragua. Kampelman also will play a leading role in trying to persuade the Senate to ratify the treaty with the Soviets to scrap medium- range nuclear missiles.

Vorontsov, a former Soviet ambassador to France, was put in charge of the Soviet delegation in Geneva after the U.S.-Soviet summit meeting in Reykjavik, Iceland, in October 1986.

His assignment was taken as a sign that the Soviets were preparing for serious negotiations. Kampelman's appointment to head the U.S. delegation was given a similar interpretation.

Negotiations are due to resume soon on long-range nuclear weapons and on defense systems. The two sides are committed to working out a treaty to reduce their strategic bombers, land-based missiles and nuclear submarines by 50 percent overall. But sharp differences over the U.S. Star Wars anti-missile program have slowed progress.

The U.S. official, who discussed the Soviet change with a reporter only if his name was withheld, said whether a treat emerges ''depends on how badly they want to deal.''

He said Gorbachev was replacing Vorontsov in order to use him even more as a trouble-shooter and apparently to show his own personal interest in Afghanistan and other regional problems.

Obukhov's experience stretches back to the negotiations that led to the 1979 Strategic Arms Limitation treaty. ''He knows the American scene,'' the official said.

He was less rigid than the Soviet military in the SALT talks, but in ''a sort of character change'' Obukhov later became more of a hard-liner while the military position eased, the official said.

Obukhov can be stubborn, but he is not emotional or confrontational, the official said. ''He knows his stuff, he's been around a long time and he apparently has their confidence,'' the official added.