MOSCOW (AP) _ The Soviet Union on Thursday protested to the U.S. State Department over what it called ''absurd allegations'' that the KGB had been using a chemical agent on diplomats for surveillance purposes.

In the first Soviet official reaction, the official news agency Tass said the Kremlin protested Wednesday's statement by U.S. Embassy officials that Americans were exposed to the tracking powder NPPD, or nitrophenylpentadienal.

U.S. officials say the invisible powder leaves a deposit on people and on objects that it touches. They said minute particles have been found, but that there was probably no immediate health risk. They did not say where the traces were found, however.

''The Soviet side resolutely denies the absurd allegations that some chemical agents are used on the staff of U.S. agencies in the USSR,'' Tass said. ''Nothing of the kind was ever practiced or is practiced in our country.''

The Soviets said that the allegation ''pursues the ends that are far removed from the interests of improving our relationships.''

The statement added the United States would be held responsible ''for the possible consequences of actions of this sort.'' It did not elaborate.

''One cannot help seeing behind the raising of this artificial question a quite definite scheme, that of preparing the ground for another slander campaign against the Soviet Union, poisoning the atmosphere in relations between our countries and fomenting enmity towards the Soviet people,'' the Soviet statement said.

U.S. Charge d'affaires Richard Combs told Americans in Moscow that several embassy employees had been exposed to the chemical tracking agent. He said he had issued a ''strong protest'' to the Soviet Foreign Ministry over its use.

Combs also briefed about 25 diplomats from embassies of U.S.-allied nations on Thursday, said embassy spokesman Jaroslav Verner. He declined to say which nations were at the meeting or what their reaction was.

In Washington, State Department spokesman Charles E. Redman, said he could ''reconfirm that everything we said yesterday is perfectly true. The evidence is there. We have absolutely no doubt in our mind that what we described as happening has been happening in the manner in which we described it as happening.''

Combs and other U.S. officials have said they could provide no details of how the use of the tracking compound was discovered, how many people may have been exposed to it, how harmful it is or where people might come into contact with it.

Redman said normal U.S-Soviet contacts were continuing.

Dr. John Baker of the American Embassy said three Americans had come into his office Thursday to ask questions about NPPD.

British political attache Simon Hemans said he attended the U.S. meeting. He quoted Combs as informing the group that ''there is no evidence to suggest some of you may have been targeted, as we have been.''

The West German and French embassies both said they could not comment on the briefing other than to say they were told about the chemical.

Combs told the American community that little is known about the substance, but that a special task force has been established in Washington to investigate it and a team would be arriving in Moscow in about 10 days to gather information.

In Washington, U.S. Sen. Strom Thurmond, a South Carolina Democrat, called the KGB's alleged use of the substance ''inhuman'' and ''barbarous.'' He added the U.S. Embassy in Moscow should be closed.

Thurmond said he planned to raise the issue with Soviet Leader Mikhail S. Gorbachev when he visits Moscow at the end of the month.