President Bush yesterday strongly backed the expulsion of more than 50 Russian diplomats and said he was "confident that we can have good relations with the Russians" even as Moscow vowed to retaliate.

"I was presented with the facts, I made the decision, it was the right thing to do," Bush told reporters on Capitol Hill.

In Moscow, Russian Foreign Minister Igor Ivanov promised an "adequate" response within hours of learning that the State Department had ordered the immediate expulsion of four diplomats suspected of being intelligence officers and had given 46 others until July 1 to leave the United States.

By late last night, the Russians had served no official notice of their intentions. But Ivanov and other senior officials indicated that Moscow would expel an equal number of Americans. Asked by CNN when U.S. diplomats would have to leave Russia, Ivanov replied: "You won't have to wait long."

Echoing a consistent theme of Russian leaders in recent weeks, Ivanov also warned against "those who are trying to push mankind and the United States back to the epoch of the Cold War."

A Bush administration official speaking on condition of anonymity said the four Russians who have been declared persona non grata and must leave within 10 days were Washington-based intelligence officers who had been involved in "handling" Robert P. Hanssen, the veteran FBI agent arrested last month on charges of spying for Moscow since 1985.

In addition, the official said, two other Russian diplomats suspected of involvement in the Hanssen case would have been expelled if they had not already left the United States. One of them was Vladimir Frolov, press attache{acute} for the Russian Embassy, who cut short his second tour of duty and returned to Moscow last week, the official said.

The Bush administration's decision to expel 46 others decisively settled a debate that has gone on in the government for years over how to respond to what U.S. counterintelligence officials say has been a large increase in the number of Russian spies with diplomatic cover in the United States. Both countries keep close tabs on foreign diplomats and, over time, develop a pretty accurate sense of the number who are involved in intelligence work, U.S. officials maintained.

The diplomatic confrontation comes at a time of increasingly testy relations between the two nuclear powers, with rhetorical clashes in recent weeks over issues ranging from the Bush administration's missile defense plans to Russia's renewed arms deals with Iran. This week, Russian officials lashed out twice at Washington for what they called "Cold War rhetoric" and for meeting with a representative of Chechen rebels.

In the two months since Bush took office, he has consciously kept Russia at arm's length, refusing to meet with Russian President Vladimir Putin until summer at the earliest and threatening to cut off financial aid to Russia's still-transitioning economy. For his part, Putin has traveled the world in an effort to drum up opposition to U.S. missile defenses and courted countries that the United States considers rogue states, including Cuba and North Korea.

The FBI has been pushing for a mass expulsion of Russian intelligence officers since detecting a substantial increase in their numbers in the mid-1990s. "They've flooded the zone," said David G. Major, a former FBI counterintelligence official. "And as long as they continue flooding the zone, it puts a strain on American counterintelligence."

Hanssen was arrested Feb. 18 after he allegedly placed a plastic bag containing secret documents at a "dead drop" under a footbridge in a Northern Virginia park. Almost immediately, White House press secretary Ari Fleischer told reporters yesterday, Bush began discussing "possible remedies" with his security advisers.

"And then last week, his national security team made a recommendation to him," Fleischer said. "The president gave the go-ahead last week. Secretary [of State Colin L.] Powell met with Russian officials last night, as you know, and that's when the action was informed -- that's when the Russians became informed of the action."

Powell told Russian Ambassador Yuri V. Ushakov of the expulsions Wednesday. Powell said yesterday that he had also "made clear" to Ushakov what "actions the Russian government needs to take to address our long-standing concern about the level of their intelligence presence in the United States."

Yesterday, Russian news agencies reported that Powell phoned Ivanov and said: "We see this issue as closed."

Bush's national security adviser, Condoleezza Rice, singled out "the Russian military presence" as being "out of line with what should be a very fruitful and, indeed, excellent relationship." Other officials said she was referring to steady growth in the number of GRU, or Russian military intelligence, officers operating in America.

The CIA traditionally has opposed mass expulsions, out of concern about the impact of Russian retaliation on its operations. But CIA Director George J. Tenet was "supportive" of this week's order because he believed that "the lack of a clear response" to the Hanssen case "would have had a negative effect," an intelligence official said.

Former senior CIA officials were divided yesterday over the wisdom of the move and its probable impact.

"This type of signal is the only signal that the Russians can understand," said Clair E. George, a former deputy director of CIA operations. "If it costs us people in Moscow, so be it."

Another former high-ranking CIA official, who asked not to be quoted by name, contended that U.S. intelligence would suffer because the number of U.S. spies in Russia is smaller than the number of Russian spies in the United States. "We're so much smaller there than they are here that I think the disruption for our side is going to be great," he said.

The expulsion of 50 people represents about a quarter of all the accredited Russian diplomats in the United States. Russia has 118 diplomats at its embassy here, 77 at its United Nations mission in New York, and five to 10 at each of three U.S. consulates.

But those numbers understate Russia's total diplomatic presence, because there are an additional 260 Russians working in the United States for U.N. agencies such as UNICEF.

Knowledgeable U.S. officials say Russia is believed to have nearly 200 intelligence officers in the United States, about the same as during the Cold War but nearly twice as many as it had in the early 1990s. The number of "illegals" -- agents without diplomatic cover here -- is unknown.

The expulsion of 50 U.S. diplomats from Moscow would make less of a dent in the overall number of Americans there. There are more than 1,000 Americans assigned to the U.S. Embassy in Moscow, including 300 diplomats, 380 nondiplomatic employees, and 300 to 400 part-time workers, according to a State Department official.

Glasser reported from Moscow. Special correspondent Colum Lynch in New York and staff writers Mike Allen, Manny Fernandez, Walter Pincus and David A. Vise in Washington contributed to this report.

Secretary of State Colin L. Powell cited "long-standing concern" about Russian intelligence activities.