The sudden resignation of Russian President Boris Yeltsin marks the end of a U.S. policy that had relied heavily on his personal rapport with President Clinton and the Russian leader's ability to manipulate the levers of power in Moscow.

While Yeltsin's handpicked prime minister and successor, Vladimir Putin, might one day develop such a relationship with an American president, U.S. policymakers were apprehensive yesterday about the no-nonsense former KGB colonel whose personal values remain unclear even to Americans who have worked closely with him.

Yeltsin, though at times exasperating to U.S. officials, had a blustery, straightforward manner--marked by scowls and bear hugs--that left little mystery about where he stood. His designated successor, by contrast, is businesslike and impersonal, "studied and serious," said one White House official deeply involved in Russia policy.

"Putin is a cool character," said another White House official. "He's smart, he's hard to read . . . he's more opaque."

This has made it hard to know whether his ascent is benign from the U.S. vantage point. "I think he recognizes the importance of the Russian relationship with the West," Clinton's national security adviser, Samuel R. "Sandy" Berger, said in an interview. "In all our dealings, the premise has been to solve problems, but the jury is out."

And the problems are many. Although Clinton and Yeltsin have smoothed their differences over Kosovo and NATO's expansion, explosive issues still threaten the U.S.-Russia relationship.

Deputy Secretary of State Strobe Talbott has been trying, in vain so far, to persuade Russia to amend the Anti-Ballistic Missile Treaty to allow the United States to build a national missile defense system. The Clinton administration is pressing for more economic reforms, further NATO expansion and tighter controls on Russia's exports of military technology. Relations also have been strained by Moscow's unrelenting military offensive in Chechnya.

The strength of the old relationship between Clinton and Yeltsin was demonstrated yesterday by a 20-minute phone call after Yeltsin's announcement. "We had our arguments. We had our fights," Clinton said later. But he added: "I liked him because he was always forthright with me. He always did exactly what he said he would do. And he was willing to take chances to try to improve our relationship."

Will Putin be willing to take such chances? Although regarded as more Western-oriented than most other candidates vying for the Russian presidency, Putin has risen from single-digit popularity to 60 percent approval ratings largely because of the war in Chechnya.

"Putin is a serious presidential candidate only because he is a master of Russian jingoism," said one U.S. official.

Putin is to some degree a familiar figure at the White House. Clinton met with him in Auckland, New Zealand, and two months ago in Oslo. Putin arrived without notes and spoke knowledgeably on various issues. But the meetings were not warm.

Berger too knows the prime minister from Putin's earlier stint as the Russian equivalent of national security adviser. But those meetings did not yield much insight into Putin's motives or leadership style, White House officials acknowledged.

Putin's background, senior U.S. officials said, is contradictory. When he served in the early 1990s as deputy mayor of St. Petersburg, a city where market and political reforms took place early and vigorously, he was known as "the go-to guy" for American companies investing there, said one senior U.S. policymaker.

On the other hand, Putin has the support of the business tycoons, or oligarchs, who have been the subject of corruption charges. He is also "a major architect of the Chechnya policy" in which Russia appears intent on crushing rebels even if it means massive civilian casualties, another U.S. official said.

A senior U.S. official for Russia policy cited some signs that Putin is sensitive to Western concerns. After Yeltsin, irritated by criticism of Russia's campaign in Chechnya, warned that "Mr. Clinton has forgotten Russia is a great power that possesses a nuclear arsenal," it was Putin who made conciliatory remarks. And after December's parliamentary elections, Putin said it was time to ratify the START II arms control treaty, which has been bogged down in the parliament for years.

Yesterday the Clinton administration played down differences with Moscow. Clinton mentioned differences with Yeltsin, but he omitted a reference to Chechnya, which had been included in the written, prepared version of his remarks. Earlier in the week, the administration did not oppose the release of a World Bank loan to Russia, though just two weeks earlier the administration indicated it might seek to block loans and loan guarantees until Moscow changed its policy on Chechnya.

But a senior administration official said Washington does not consider Putin's electoral victory a given. "If things begin to go badly in Chechnya [for Russian forces], the same wave he's riding now could engulf him," the official cautioned.

Berger added that the weeks leading up to the March election are an especially delicate time. "As Nelson Mandela has said, the second democratic election is the most important" in testing a nation's progress toward democracy, Berger said.

Over the past week, senior White House officials had received sketchy intelligence reports that Yeltsin might step down. The White House reaction: a shrug of the shoulders. Similar rumors that Yeltsin was near death or retirement had made the rounds for the past two years, they said.

This time it was true. Around 4 a.m. yesterday morning, the White House situation room rousted Berger to report that Yeltsin really was bowing out. Berger, in turn, woke up Clinton with a phone call shortly after 5 a.m.

Later Clinton heaped praise on Yeltsin--crediting him for leading the Russian people as they confronted the "unprecedented challenge of building a new democracy and a new life after decades of corrosive communist rule." But that only hinted at the complex relationship the two leaders have had.

Clinton, several aides said, is genuinely fond of Yeltsin. He remains grateful that Yeltsin met with him in June 1992, when Clinton was running third in the presidential campaign that year. For a time, Clinton and aides thought this personal rapport could be the core of a solid U.S.-Russian relationship.

But in recent years, Yeltsin's popularity declined. Corruption stymied economic reform and led to rifts over aid from the International Monetary Fund and World Bank. Yeltsin was unable to win parliamentary approval of START II.

And as Yeltsin's health has steadily declined, the communications between Clinton and Yeltsin have become less frequent and far more stilted and formal in tone. In recent years, U.S. officials have said previously, intelligence reports suggested Yeltsin regularly had no more than 30 minutes or so of productive working time each day.