WASHINGTON--The Reagan administration, saying it has a strong diplomatic signal from Moscow, is about to call for a special meeting of U.S. and Soviet negotiators to try to nail down an agreement to reduce nuclear missile warheads in Europe to 100 on each side.

The Geneva session, which an administration official said may be held as early as Friday, will test whether Soviet leader Mikhail Gorbachev's attempt to sidetrack the U.S. "Star Wars" program in Iceland will carry over to the negotiating table.

Both superpowers indicated yesterday they were ready to build on the Reykjavik summit and move toward an agreement sharply reducing U.S. and Soviet nuclear weapons in Europe.

While President Reagan declared in a Baltimore speech, "Let's not look back and place blame," U.S. arms specialists worked on a package of proposals for Max Kampelman, the chief U.S. negotiator, to present to Viktor Karpov, head of the Soviet delegation in Geneva.

Either side can request a special session in the Swiss city and it is customarily granted.

The main U.S. objective will be to formalize Gorbachev's apparent acceptance in Iceland of Reagan's proposal that Soviet intermediaterange warheads in Asia by reduced to 100 from the current level of 513.

The U.S. official, who spoke only on condition of anonymity, said the Soviets had indicated through diplomatic channels they were ready to separate the missiles issue from their demand at the summit that Reagan constrain the U.S. Strategic Defense Initiative.

"We're going ahead," the official said.

Confirmation of the Soviet signal appeared to come from Bonn, West Germany, where Karpov said his government was ready to talk about removing medium-range nuclear missiles from Europe without linking the issue to Star Wars.

"We are ready to discuss and solve the question separately if there is willingness on the part of the United States," the Soviet negotiator said.

But there was a conflicting signal from Moscow. Argentine President Raul Alfonsin, after meeting with Gorbachev on Wednesday, quoted the Soviet leader as saying the proposals were not divisible. That would include Gorbachev's demand about Star Wars.

The Soviet warheads in Asia, targeted on China, Japan and other countries, were considered the last major roadblock to a Euromissile treaty.

The agreement would impose a ceiling of 100 U.S. and Soviet warheads on the continent. An additional 100 U.S. warheads would be held in reserve at home to match the Soviets' Asian arsenal.

Other post-summit developments yesterday:

.The State Department questioned the sincerity of a Soviet decision to withdraw six regiments from Afghanistan. New arms were shipped in recently, and four of the regiments were not in combat. If all of them left, there still would be more than 110,000 Soviet troops in the country, spokesman Pete Martinez said.

.The last of 25 Soviet diplomats expelled from the United States under a broad accusation of spying have left the country, the State Department announced. A U.S. official, speaking on condition of anonymity, said that among the final five to depart were Valery Savchenko, said to be the KGB station chief in New York, and Vladislav Skvortsov, said to be the New York chief of the GRU, Soviet military intelligence.