The United States will act as a "full partner" with Egypt and Israel at the Camp David summit on the Middle East next month rather than being a "non-interested mediator or message-carrier," President Carter said yesterday.

Asserting that failure could lead to new conflict and "severely damage the security of our country," Carter pointed in his news conference remarks to an activist role for himself at the Sept. 5 meeting with Egyptian President Anwar Sadat and Israeli Prime Minister Menachem Begin.

Begin and Sadat are coming without having given him any commitment to change their strongly conflicting positions, the president added. "We do not have any assurance of success," he said. "I do not anticipate being completely successful there and having a peace treaty signed" at the open-ended Camp David meetings.

But Carter also noted that in accepting the invitations carried to them last week by Secretary of State Cyrus R. Vance, the two leaders had agreed to come to Camp David "with flexibility and an ability to act."

In describing the American role at the summit as that of a "fuller partner," Carter was knowingly choosing a code-word in Middle East diplomacy. But he appeared to be trying to invest it with a new and less controversial connotation.

Since his visit to the United States last February, Sadat has frequently called on the United States to drop its role as a mediator in the Middle East and to become a "full partner" with him in getting Israel to withdraw from all Arab territories occupied in the 1967 Arab-Israel war as a precondition for peace.

Israel wants nothing more than a mediator's role for the Carter administration.

The Egyptian leader said last week he had accepted the invitation because it was proof that Carter was ready now to be a full partner. Speaking with Sadat at the same term but carefully avoided spelling out the American interpretation of it.

Carter also skirted any detailed discussion of the summit yesterday, but he did pave the way for an active American role in presenting ideas to the two leaders to resolve their conflicts by pointing to what he sees as the big risks he personally is taking and the enormous stakes involved in the Middle East for the United States.

"It is a very high risk thing for me politically," the president said. "Now if we are unsuccessful at Camp David I will certainly have to share part of the blame. But I don't see that I could do anything differently" because continuing deadlock might produce a new Middle East war, he added.

Vance and his Middle East specialists have spent the week working out a detailed set of options for the president to carry with him to Camp David and to interject to keep the negotiations moving should they become bogged down. Carter may hear of these options at his regularly scheduled weekly breakfast with Vance today.

Carter said he decided entirely "on my own" to issue the invitations and told his foreign policy advisers of it later.

On another foreign policy issue, Carter reiterated his commitment to "normalizing" diplomatic relations with China but did not give any timetable. He noted "a new impression that I have of the leadership in China" being "more outreaching, more outgoing" and promised to "respond in good faith."

Asked if he intended to continue selective restraints on trade with the Soviet Union if allied countries declined to join in his efforts, Carter responded by stressing that export refusals would be judged on a case-by-case basis.

"We obviously don't have an inclination to declare a trade embargo against Soviet Union," he said. "It is in the advantage of our country to have trade with the Soviet Union."

France declined this week to heed a Carter administration appeal to ban shipment to the Soviet Union of a computer similar to an American computer Carter banned for export to Moscow last month.