President Carter and the leaders of the United States' principal European allies assured the Soviet Union today that they will not allow the new U.S. relationship with China to endanger Western relations with the Soviets.

Ending their informal, two-day summit conference on this French island in the Caribbean, Carter, French President Valery Giscard d'Estaing, British Prime Minister James Callaghan and West German Chancellor Helmut Schmidt each stressed the importance they attach to a policy of detente with the Soviets and especially to reaching a new strategic arms limitation (SALT) agreement.

Callaghan and Schmidt also issued direct, personal appeals for swift approval by the U.S. Senate of a new SALT II accord, which Carter reassured the others, according to Callaghan, is in the "final stages" of negotiation.

The concluding public statements by the four leaders illustrated how this conference of North Atlantic allies was in many ways dominated by the unseen presence of two non-Atlantic nations -- China and especially China's chief rival, the Soviet Union.

Carter called China's recent openings to Western nations "constructive" and pledged that the U.S. decision to establish diplomatic relations with the Chinese "will never be an obstacle to detente."

He added, without explanation, that he hoped a closer U.S. relationship with China might in the future result in a "strengthening of the ties of friendship with the people of the Soviet Union."

The four men, dressed in coats and ties for the first time since they arrived on this resort island Thursday, ended their meetings with prepared public statements spoken from a grassy area dotted with palm trees within sight of the Caribbean Sea.

The leaders went out of their way to stress the friendly atmosphere of their talks and their personal relationships, and in their statements Carter and his European counterparts each gave the other what they wanted.

Carter faces a bruising battle in the Senate to win approval of a SALT II accord and could only welcome the strong support for the treaty from Schmidt, Callaghan and, less directly, Giscard.

For their part, the Europeans are extremely sensitive to any measures that might upset detente and increase the Soviet threat to Western Europe. As a result, Carter's decision last month to establish diplomatic relations with China was an important topic here, with Carter's pledge not to allow that to interfere with Soviet-American relations a reassuring balm to the others.

The European leaders left no doubt about the importance they attach to relations with the Soviet Union.

Callaghan called European relations with the Soviet Union "central" to the countries involved and said increased Western contact with China -- such as the impending sale of jet planes to China by Britain that he announed here -- will not be done "at the expense of any other nation."

Callaghan was also the most explicit in urging approval of a new SALT treaty."It would be a very sad day if this agreement were not ratified," he said.

As predicted, the summit ended with no agreement or public policy decisions after a final meeting of more than three hours today that dealt with a number of trouble spots around the world.

These included the Middle East, Cambodia, Iran, Pakistan and Turkey, where a deepening economic crisis threatens the stability of a pivotal member of the North Atlantic Treaty Organization.

Giscard, the summit host, described the talks as "frank, friendly and useful," and said the four leaders at their meeting today also emphasized improving relations with Third World nations.

White House press secretary Jody Powell said later that Carter considered the summit to be "extremely useful and very positive" and that he thought that opinion "is shared by his colleagues."

Aside from Soviet-American relations and the new opening to China, the status of Iran, an important supplier of oil to the four countries, was probably the most important topic discussed here. It remained unclear what, if anything, was decided about the deterioriating situation there.

There have been times that Carter's relationships with some of the others, particularly Schmidt, have been described as strained. If that was the case here, it was not evident from the public statements of the four, whom Callaghan described as the best of friends among whom differences of emphasis are of little consequence.

Carter said the talks took place in an atmosphere of "almost unprecedented harmony." He said he had never attended a conference "that was more beneficial to me and more substantive in nature."

The final gathering of the four men here was a lunch they attended with their wives after the public statements.

Later today, Carter and Callaghan attended a reception hosted by Giscard for French members of parliament from Guadeloupe. By then, Schmidt had already left the island.

Carter is to remain here until Tuesday and is expected to work on domestic economic matters during the vacation. Late today, he went scuba diving for the first time, according to Powell.

Asked whether the president knows how to scuba dive, the press secretary replied, "God, I hope so."