Rarely in the history of U.S.-Russian relations have the leaders of the two world powers talked to each other so warmly and so often on the phone -- yet trusted each other so little.

President Clinton calls President Boris Yeltsin regularly. Vice President Gore's aides boast of his warm chats with Prime Minister Sergei Stepashin and former prime minister and Balkan intermediary Viktor Chernomyrdin. And Secretary of State Madeleine K. Albright is in constant contact with Foreign Minister Igor Ivanov, who embraces her warmly, and with Chernomyrdin, who affectionately calls her "mother boss" in Russian, according to U.S. officials.

Yet mistrust rather than friendship has dogged diplomacy between the United States and Russia over the past week, as the Clinton administration worried that Russia would try to undermine the settlement in Kosovo that it had helped bring about.

The sudden, unexpected deployment of Russian troops to the Kosovo capital of Pristina early yesterday, in advance of the international military force (KFOR), has added to the mistrust and tension between the two countries, especially since it came hours after Ivanov promised Albright deployment would not take place before an agreement over Russia's role in KFOR.

"It's not a question, when Ivanov makes a statement, of doubting that individual," said a senior White House official, "but seeing so many actors and the lack of coordination between those actors makes people here wonder whether we can count on them." The question of trust, he said, was "not so much of a personal issue" as it was one of the "consistency" among different Russian leaders.

The fragility of the U.S.-Russian relationship has been highlighted by the fact that only a week after Deputy Secretary of State Strobe Talbott was hailing Russia for playing a constructive international role, he rushed to Moscow to defuse a crisis while British forces confronted Russian and Serb troops at the airport in Pristina.

The abrupt change in diplomatic fortunes highlighted the perils of the U.S.-Russia relationship. One week the two countries seem close to breaking out champagne, the next their soldiers are eyeball to eyeball.

Critics say the reason is that the Clinton administration policy is misguided -- relying too heavily on personal relationships with a leadership that is unsteady at best and, in the case of Yeltsin, destructive of democratic institutions that could have outlived him. And they say that the Clinton administration has pushed Russia too hard by expanding NATO and prosecuting a war in Kosovo over Moscow's objections, while failing to foster a lasting basis for relations with Russia.

"What will Mr. Talbott be remembered for?" said Dmitri Simes, head of the Nixon Center for Peace and Freedom. "The defeat of Serbia, or a realignment of Russia and other nations against the United States and the allies?"

Senior Clinton administration officials say, however, that they are doing the best they can with a fractious, but democratically elected Russian government, and that Yeltsin's most recent change in prime ministers leaves Russia's government more willing to work with the West than it was before the war.

"They are going through a very difficult period," Albright said on her plane while returning Friday from a four-day meeting with foreign ministers of six other major industrial nations and Russia. "They are trying to sort themselves out, what their own agenda is in the world." She added that "it is remarkable that . . . with our number one enemy for 50 years, we now have a very important, serious working relationship dealing with the most complicated issues that exist."

"We're both great powers with global interests and global roles. What we're trying to do now that we no longer have ideological division between us is to see if there are ways that were not possible during the Cold War to maximize our agreements and minimize our differences," Talbott said recently.

Yet serious differences remain. While Albright's four days in Cologne displayed the intensity of contact between U.S. and Russian officials, they also showed the lack of U.S. trust in Russian commitments.

Just two days after getting Belgrade, with Russia's help, to agree to NATO terms, U.S. officials were uncertain whether Russia would show up at Cologne, Albright said. When Ivanov raised 20 objections to a 33-paragraph proposed draft of a U.N. Security Council resolution, they were unsure Russia would negotiate in good faith.

"We can't make our entire strategy subject to a Russian veto," fumed a senior British official early in the week.

Once the text of the resolution was settled, the foreign ministers again reverted to mistrust. At the United Nations Russia resisted finalizing the language, and U.S. officials feared that Russia secretly planned to let the U.N. resolution languish. That would have blocked deployment of the international military force, created fears of a "security vacuum" and given Serb forces an excuse to halt their withdrawal from Kosovo, senior U.S. officials said.

That anxiety led to the insertion of a sentence in the military-technical agreement linking it to a vote on the U.N. resolution and deployment of KFOR soldiers. The sentence offended Ivanov, who said it showed that Russia wasn't trusted. After Russia agreed to resolution language in New York, the offending sentence was deleted, but Russian feelings were bruised.

Russian leaders were also angered that NATO refused to give Russia a "zone" of its own in Kosovo, while handing out areas to other members of the international military force. Albright and others urged Russia to accept a role similar to the one it played in Bosnia -- where its troops report to a U.S. commander -- but Russia felt it deserved more for its mediation efforts. NATO allies suspected Russia would collaborate with Serbs if given its own zone, subverting NATO aims and possibly leading to a partition of Kosovo.

On her way back to Washington, Albright was saying that the week had reinforced the U.S.-Russian relationship and said her ability to phone Ivanov from Macedonia to discuss Russian troop movements was an example of the virtues of modern cell phone diplomacy.

Albright was unaware that as she spoke, Russian troops were rolling toward Pristina. Within 24 hours, her exercise in cell phone diplomacy had to be rescued by some larger appliances like the Apache helicopters and British tanks that rushed to the Pristina airport to outnumber the Russian contingent.

This week, Clinton and Yeltsin are to meet at a summit in Cologne along with the leaders of six other major industrial nations. Clinton plans to call Yeltsin today, the White House said. The Russian leader's shaky health and unpredictable nature -- simultaneously helping and denouncing NATO, constantly reshuffling his government -- embody the nature of the U.S.-Russia relationship at a time when Russia's financial institutions are in ruins, the rule of law has been corrupted, civil society is weak and the constitutional process undermined.

Yet Russia's dire but self-inflicted woes will get less urgent attention than Russia's role in the fate of Kosovo. Administration officials say that they will press ahead. "The assurance she received yesterday was not correct," State Department spokesman James P. Rubin said of Albright's talk with Ivanov. "She wants to move beyond that."

CAPTION: Deputy Secretary of State Strobe Talbott takes a call near Foreign Ministry in Moscow, where he is negotiating Russia's role in Kosovo peacekeeping.

CAPTION: Serb forces withdrawing from Kosovo flash three-finger salute as they pass peacekeeping troops arriving at Kacanik. Earlier they cheered Russians' entry.