The Supreme Court refused yesterday to block the extradition to Israel of John Demjanjuk, a retired Cleveland auto worker accused of being "Ivan the Terrible," the gas chamber operator at the concentration camp at Treblinka, Poland, where 900,000 Jews were killed during World War II.

Demjanjuk, 65, is the first alleged war criminal approved for extradition to Israel since the United States and Israel signed an extradition treaty in 1963. He faces trial there and possible execution.

Following the court decision, the State Department, which is required to approve the extradition, yesterday authorized the Justice Department to proceed. And Justice Sandra Day O'Connor denied a last-minute request to delay the extradition. The actions removed the last possible barriers.

The only other major Nazi war criminal to be tried in Israel was Adolf Eichmann, who was kidnaped by the Israelis from Argentina in May 1960. He was hanged in Jerusalem on May 31, 1962.

Demjanjuk, confined at a federal prison hospital in Springfield, Mo., since last April, has denied the allegations against him.

Neal Sher, head of the Justice Department's Office of Special Investigations, which investigates suspected Nazi war criminals, said yesterday that the Demjanjuk case has been a priority since the office was created in 1979.

"This shows the government is serious and it sends a signal to all the people [Nazi war criminals] who are still out there. The passage of time doesn't diminish what they've done," Sher said.

Following a probe by the Office of Special Investigations, Demjanjuk lost his citizenship in 1981 because he lied about his past when he emigrated to the United States in 1952 under the Displaced Persons Act. He became a citizen six years later in Cleveland.

Demjanjuk had unsuccessfully appealed his citizenship revocation, along with a later deportation order to the Soviet Union. But the subsequent request for extradition by Israel took precedence over the deportation order.

Murray R. Stein, associate director of the Justice Department's Office of International Affairs, who handled the extradition work for Israel, said the case presented difficulties because "Demjanjuk was being extradited to a country that didn't exist when the crimes occurred . . . . The victims had nothing to do with Israel. The crimes occurred in Poland."

But he added that the extradition is legal under the U.S. treaty with Israel. At an extradition hearing last March, the government presented affidavits from Holocaust survivors identifying Demjanjuk as "Ivan the Terrible" and describing his activities at the death camp in 1942-43. "Ivan had a weapon, a pipe, a sword, a whip, and he tortured [Jews] before they went into the gas chamber," said an affidavit from Elijahu Rosenberg, a Treblinka survivor. It said that "Ivan" crammed hundreds of people into the gas chambers and then turned on the motors for the poison gas.

Demjanjuk, a Ukrainian who served in the Soviet army, said he was captured by the Germans and was a prisoner during World War II.

Federal officials said yesterday that the State Department is expected to move rapidly and that Demjanjuk could be turned over to the Israelis this week, perhaps as early as today.

Demjanjuk would be only the second person extradited on war crime charges since the Office of Special Investigations opened in 1979. Earlier this month, accused Nazi war criminal Andrija Artukovic, who allegedly ordered the deaths of Jews, Serbs and gypsies in Croatia, was sent to Yugoslavia for trial.