President Clinton won assurances Thursday in Kremlin talks with Boris Yeltsin that Russia will not retreat from tough reforms. In a goodwill gesture, the leaders will seal an agreement Friday to stop targeting nuclear missiles at one another, U.S. officials said.

In a tense moment in Russian history, Clinton found Yeltsin "very much on top of his game" and "firmly in control," Secretary of State Warren Christopher said.Yeltsin, in fact, is not only committed to economic reform but prepared to accelerate it, Treasury Secretary Lloyd Bentsen said at a joint news conference with Christopher.

"President Yeltsin emphasized there is no turning back from fighting inflation and continuing to privative," Bentsen said, noting that Russian inflation had dropped from 30 percent to 12 percent a month.

Opening three days of talks, Yeltsin and Clinton strode from opposite ends of the vast Hall of St. George in the Kremlin, meeting in the center of the gleaming white and gold room for a warm handshake.

Clinton, hoping to steel Yeltsin against rising opposition to his economic and political reforms, said the Russian president "deserves our support."

After their first round of talks, Clinton did some sightseeing in Moscow, visiting a rebuilt Russian Orthodox church that the Communists had razed, chatting with workers in a market and strolling though Red Square.

In the church, Clinton went to an altar for lost loved ones and said a prayer for his mother, who died last week. He also lit a candle.

Talking about his meeting with Yeltsin, the president said: "The whole conversation was about how our work together can help change the lives of ordinary Russian people for the better."

Clinton said he discussed with Yeltsin ways to speed up international aid and "a big joint American-Russian project on energy" that he said would create many jobs.

Clinton said he presented Yeltsin with a map showing where in Russia different types of Western aid had gone "so we can document what we have given and where it has gone."

The summit talks coincided with the rise of ultranationalists and a communist comeback. Christopher said Yeltsin was not cowed by his political rivals. He said Yeltsin gave Clinton a thorough appraisal of the political turmoil here.

"It's reassuring to be back here in the presence of the Russian leaders and find them undeterred by the events, indeed redoubling their efforts to move forward with reform process both from the economic side and on the political side and . . . on the foreign policy side," Christopher said.

Clinton told Yeltsin that Russia has to take steps to "ease some of the hardships" imposed on Russian citizens by tough inflation-fighting measures. In that regard, Clinton has promised to help.

At the outset of the summit, Yeltsin said he and Clinton would build on "the personal rapport and friendship" that developed during their first summit, last March in Vancouver, British Columbia.

"I believe that together, we can work to lead a new security for Europe based on democratic values, free economies, the respect of nations for one another," Clinton told Yeltsin.

Clinton's message was that despite the rise of Russian ultranationalists, communists and other foes of reform, Yeltsin must not relax the pace of painful change.

Ultranationalist Vladimir Zhirinovsky, a leader of Yeltsin's foes, sniped at the summit from parliament.

"It's nothing good from Mr. Clinton to Russia, nothing for Russia," Zhirinovsky said Thursday. "Only promises, promises: `We'll give you money, money.' Where? Where is this money? `We'll give you technology.' Where is this help? We need no help from the United States and the West . . . We are a very rich country, very strong, very rich."

On Friday, Clinton and Yeltsin will finalize an agreement to stop aiming long-range missiles at each other, administration officials said. It will be announced as part of a ceremony with Ukrainian President Leonid Kravchuk, who has agreed to dismantle his country's nuclear arsenal for security guarantees from the United States and Russia and a doubling of U.S. aid.

Yeltsin gave Clinton a brief sightseeing tour of the Kremlin's Cathedral Square. Later, Clinton browsed at an outdoor kiosk near the former KGB headquarters at Lubyanka Square. He went into a bakery and a meat and fish shop. Taking off his gray fur hat, Clinton waved to a cheering crowd of more than 500 people.

Moscow was the centerpiece of Clinton's eight-day European trip, which began with a NATO summit and a meeting in Prague with Eastern European leaders eager for closer ties with the Western military alliance as a safeguard against any aggressors.

A light snow and 8-degree temperatures greeted the president as he arrived in Moscow at 1 a.m. on a flight from Kiev, Ukraine. Despite the late hour, he worked out in his hotel gym before going to sleep.

Clinton had detoured to Kiev to help Kravchuk sell to a reluctant Ukrainian parliament the agreement to scrap the country's arsenal of 1,800 nuclear warheads.

Kravchuk said Ukraine "will not stand in the way" of denuclearization, but lawmaker Les Tanyuk said Kravchuk's views are "in opposition to parliament's position."

The Clinton-Yeltsin summit is the third meeting between the two leaders and it comes after a tumultuous year in Russia in which Yeltsin used tanks to overcome opponents who defied his order to disband parliament. That was followed by the rise of ultranationalists and communists in December's parliamentary elections.