North Atlantic allies of the United States today hailed the first summit meeting in six years between U.S. and Soviet leaders as a diplomatic success that forged a new dialogue between the superpowers and enhanced the climate of East-West relations.

The heads of most western allied governments, who assembled at NATO headquarters to hear President Reagan's account of his talks with Soviet leader Mikhail Gorbachev, emerged from the 80-minute meeting expressing satisfaction that a personal rapport had been established between the two that could help to ease distrust between Moscow and Washington.

"It's a new basis for confidence in the future," said British Prime Minister Margaret Thatcher. "We were very pleased by what we heard from the president, and we offered our congratulations, our support and our appreciation."

Reagan stopped in Brussels today for less than three hours before resuming his return trip from Geneva to Washington. Among the 16 NATO members, only France, Spain and Greece were not represented by their heads of government, choosing instead to send foreign ministers.

As he arrived at the sprawling NATO compound, Reagan was asked by reporters if he felt the summit were a success. He remarked that "neither side got everything they wanted, but I think we made great progress."

In the closed-door session, Reagan gave a 20-minute assessment of his meetings with Gorbachev before opening the floor to queries and comments.

Shortly after his arrival at the NATO complex, a bomb went off about five miles away at the European headquarters of an American firm, Motorola Data Systems. Police said there were no casualties. Responsibility for the blast was claimed by the Fighting Communist Cells, which has claimed previous attacks against NATO targets.

According to several leaders, Reagan laid particular emphasis on his impressions of Gorbachev and the relaxed personal relationship they appear to have cultivated.

Italian Prime Minister Bettino Craxi said Reagan noted that he found Gorbachev "more flexible and more reasonable" than he had expected. The Soviet leader also reportedly conveyed an aura of authority that left no doubt about the primacy of his power.

Most of the European leaders said that the summit's favorable outcome was assured simply by fostering a more civil superpower dialogue. But they also stressed that a more positive atmosphere alone could not be expected to bridge serious differences over curbing nuclear and space weapons.

"Geneva is not the end of a process but, we hope, the beginning of a new and more constructive stage," declared the NATO secretary general, Lord Carrington.

"Our expectations were not deluded," said Craxi. "The summit was intended to rebuild trust, and this objective was met."

Chancellor Helmut Kohl of West Germany proclaimed that "a new phase of regular East-West dialogue" may have been launched.

The Bonn government's efforts to establish closer contacts with East Germany have been thwarted by prolonged tension between Moscow and Washington, but Kohl indicated that relations with East Berlin are bound to improve. "I always believed that the larger flow of water carries the small stream with it," he said.

Despite the caution underscored by the failure to narrow differences on many key issues, allied leaders said they were heartened by some specific points of accord in today's joint summit statement.

In addition, Western European leaders said they understood that Gorbachev may have revived hopes for compromise over the U.S. Strategic Defense Initiative research program into a ballistic missile defense system.

"Ronald Reagan will not trade away his rights to conduct research in this area, and the Soviet Union appears to have accepted the fact that a ban on research is not realistic," said Norway's Prime Minister Kaare Willoch.

Allied leaders cited the willingness to reach a separate agreement on medium-range missiles in Europe, independent of the negotiations on strategic and space weapons, as an encouraging sign of potential progress in reducing the arsenals that have generated the most public alarm in Europe.

European governments also were pleased by a reference in the joint statement that Moscow and Washington vowed "to accelerate efforts" to conclude an international convention banning chemical weapons.