With the zest reserved for exceptional propaganda opportunities, the Kremlin today seized on Egyptian President Anwar Sadat's disclosure of arms shipments to Afghan rebels as proof that the United States is conducting an undeclared war against Afghanistan.

The government newspaper Izvestia described Sadat's disclosure as a rare act of "political striptease" that laid bare a conspiracy coordinated by the CIA.

"This fresh evidence," the press agency Tass declared, "should be enough to convince the U.N. General Assembly that if anyone was interfering in Afghan internal affairs it was the United States.

In an interview with NBC broadcast in the United States Tuesday night, Sadat said that immediately after the Soviets invaded Afghanistan in December 1979, the Carter administration asked that Egypt provide Soviet-made weapons to resistance forces. Sadat said that the United States had been "very generous" in paying for these weapons.

Izvestia went into raptures describing the "facts" surrounding "the undeclared war that reactionary forces are waging against the Afghan people." One fact ommitted was that the weapons involved were Soviet-made and given to Egypt before Sadat broke with Moscow in 1972.

Izvestia talked about the killing of innocent people, bombing attacks on schools, hospitals and other buildings and about firebombings of public buildings, "including mosques."

Saying that these "facts speak for themselves," the commentary concluded that "the hands guiding the war" in Afghanistan are in Washington, which is plotting with China and "unspecified other client states" against a legitimate government.

Tass said that subversion against the Kabul government is coordinated by the CIA and involves Egypt, Saudi Arabia, China and Pakistan. It said "bandits" had received more than $100 million in arms and that the Reagan administration "has ordered a considerable increase in these supplies."

"In the light of these new facts about the buildup of U.S. interference in Afghanistan affairs," Tass said, "how hypocritical are the appeals from Washington for a withdrawal of the limited Soviet contingent there."

The Soviet Union is estimated to have 85,000 troops in Afghanistan.

Tass described China and Egypt as suppliers of weapons that are paid for by the United States and Saudi Arabia and sent by air and land to Afghanistan, with Pakistan providing "unhindered use of its territory."

In a related effort to counter recent allegations by Secretary of State Alexander M. Haig Jr. that Soviet chemical weapons were used in Afghanistan, the Russians gave prominence to a report in the West German newspaper Suddeutsche Zeitung.

The newspaper quoted an International Red Cross representative, Jean-Michel Monod, as saying that the allegations were an "irresponsible calumny."

"None of the Red Cross physicians working in Pakistan border areas has ever met patients affected by the use of chemical weapons," Tass quoted Monod as saying. Tass said Edgar Frey, a physician from Zurich, had confirmed this, too.

Moscow is likely to use Sadat's NBC interview and the West German report to try to easeAfghanistan's diplomatic isolation and to press for a political solution that would legitimize the Babrak Karmal government there.