For the first full month of the air war against Yugoslavia, when President Clinton was constantly working the phones to maintain international support, one critical leader was not on his call list: President Boris Yeltsin. White House officials said there was no point in talking, so strained had U.S-Russian relations become over the NATO bombing.

Yet this week, during the closing days of the war, the line between the White House and Moscow has been humming. Clinton called Yeltsin twice in as many days on Monday and Tuesday; Vice President Gore called the Russian peace envoy, former prime minister Viktor Chernomyrdin, at midnight (EDT) Monday.

The calls, senior administration officials said, were essential to untangling a knot of disagreements between NATO and Yugoslavia that threatened to undermine the peace terms that Yugoslav President Slobodan Milosevic agreed to last week.

Just months ago, senior administration officials were commonly disparaging Yeltsin as an enfeebled and often besotted leader who was abandoning reform and was only capable of a half-hour or so of lucid work per day. These days, the Clinton line is again -- as in earlier days of the administration -- to tout Yeltsin as the virtually indispensable man.

Clinton appealed to Yeltsin not to give "Milosevic any room for comfort," as one administration official put it. Despite Russia's historical relationship with the Serbs of Yugoslavia -- and its anger over what it views as NATO's illegitimate war against a sovereign nation -- Yeltsin apparently decided he wanted an end to the conflict on terms that would allow Moscow to take part of the credit.

White House officials who listened in on this week's Clinton-Yeltsin phone calls said they were general, not a precise detailing of the complicated political and logistical issues that needed to be resolved in advance of last night's agreement between NATO and Yugoslav commanders. Even so, Clinton aides said they were pleased that -- unlike some occasions in the past -- Yeltsin was engaging in a back-and-forth dialogue, rather than merely reading from a script.

And they gave Yeltsin credit for pushing his foreign minister, Igor Ivanov, to reach an agreement at a Group of Eight meeting in Germany Tuesday on the text of a U.N. Security Council resolution for ending the war. These talks, in Cologne, had been stalled on Monday, with Ivanov telling other foreign ministers, according to U.S. sources, that he did not have instructions from his government.

By Tuesday, he apparently had them. In the end, Russia agreed to either yield or find rhetorical compromises on a number of points that had snagged the negotiations. These included language stating that the United Nations would pass a so-called Chapter 7 resolution, allowing robust rules of engagement for peacekeepers.

The resolution also pledges that withdrawal of Yugoslavia's military and special police forces will be synchronized with arrival of international peacekeepers; this language is meant to address Russian concerns about a vacuum during which Serbs living in Kosovo would be at risk from the majority Albanian population there and the rebel Kosovo Liberation Army.

The agreement on these details allowed NATO to step around the larger question that has been impeding an end to the war since Milosevic broadly accepted the alliance's terms a week ago: In what sequence will the warring parties walk toward peace? Essentially, by compressing three events into rapid progression -- a Yugoslav withdrawal, a U.N. resolution and a bombing halt -- the three sides were able to make irrelevant a dispute and which should come first.

There remains a pivotal detail still to be resolved: What role will Russia play in the international peacekeeping force set to take over in Kosovo? The United States has insisted that, while it wants Russian involvement, the force must have NATO at its core. Russian forces want their own role outside NATO command.

Deputy Secretary of State Strobe Talbott was on his way to Moscow last night to try to find a compromise. White House officials reiterated yesterday they will never agree to anything that gives Russia command of its own sector of Kosovo, which might pave the way for an ethnic partition of the embattled Serbian province.

For Clinton, who has come under intense criticism for letting relations between the United States and Russia on arms control and other areas drift in recent years, the question is what the recent cooperation over Kosovo means for the larger relationship. Robert Zoellick, who served in the Bush administration State Department, said that for the typical "informed Russian, Kosovo remains a sore point."

"The Russians are out for their own interests, and they will look for opportunities to push them," Zoellick said. "It is difficult when they're out, but it's also going to be difficult when they're in."

Administration officials said the past week showed that Clinton and Yeltsin still have enough rapport to work effectively. But they acknowledged that it also showed how much depends on the fate of a politically and physically frail leader in Moscow.

Deputy national security adviser James B. Steinberg said Kosovo has caused strains in the relationship, but said the past week has also highlighted the potential. "It may be a case of taking one step back to take two forward," he said. "It has shown both sides the value of the relationship as well as the resilience of the relationship."

CAPTION: President Clinton's talks with Boris Yeltsin have been essential to untangling a knot of disagreements between NATO and Yugoslavia that threatened to undermine the peace terms, according to senior administration officials.

CAPTION: The White House, after initially ignoring him, is touting Russian President Yeltsin as indispensable.