The State Department cautioned Congress yesterday not to expect an imminent breakthrough in the long-stalled Middle East peace process as the Palestine Liberation Organization announced strong opposition to Egyptian President Hosni Mubarak's proposal for direct Arab-Israeli negotiations.

Richard W. Murphy, assistant secretary of state for Near Eastern and South Asian Affairs, told a House Foreign Affairs subcommittee that the United States is ready to "re-engage" in the peace process.

But he warned that "many questions" remain unanswered and that "a lot of clarifying" is necessary before the current flurry in Egyptian and Jordanian peace proposals and the recent accord between Jordan's King Hussein and PLO chairman Yasser Arafat can be assessed.

Murphy's warning was a continuation of what a White House official described Tuesday as administration concern that expectations about any immediate resumption of Mideast peace talks need to be lowered.

Murphy indicated that the administration has not decided what action, if any, it can take to further the process and, before deciding, is waiting to see what emerges from conflicting proposals by Egypt and Jordan.

While Mubarak has said he favors direct negotiations, Hussein has proposed an international conference on the issues. During a break in hearings on the administration request for about $1.2 billion in economic assistance for Middle Eastern and European countries for fiscal 1986, Murphy said greater U.S. involvement is contingent on a clear Arab commitment to U.N. Resolution 242. "Without that, the peace process is not going to move ahead," he said.

That resolution calls for Israel to return lands it occupied in the 1967 Arab-Israeli war in return for peace and a promise of secure borders.

Asked about Mubarak's proposal that the United States hold preliminary talks with a joint Jordanian-Palestinian delegation, Murphy replied that "the name of the game" remains direct Israeli-Arab talks. He added that the United States is not interested in "prenegotiating our position" with the Arab side in peace talks.

Panel members asked Murphy repeatedly what the United States is doing to facilitate the peace process at a time when Israeli and Arab leaders are showing renewed interest in it.

Murphy replied vaguely, saying several times, "When the parties are ready, we're ready" to play a more active role "in any way that seems appropriate at the time."

His cautious comments came as the PLO issued a statement from its headquarters in Tunis rejecting Mubarak's call for negotiations leading to direct talks between Israel and a Jordanian-Palestinian delegation.

At the same time, Israeli Foreign Minister Yitzhak Shamir took issue with Mubarak's suggestion of preliminary talks in Washington involving an Arab delegation but not Israel. Shamir called it "a transparent attempt" to open contacts between the United States and the PLO.

A PLO spokesman, rejecting the idea of direct talks, said that his organization "considers that an international conference under United Nations auspices is the only proper framework for finding a just solution to the Palestinian question," and that it would insist on participating "on an equal footing" with other participants.

The statement again highlighted continuing uncertainty about PLO agreement on a peace formula, an issue first raised by the open attack of hard-liners within the organization on the accord reached Feb. 11 between Hussein and Arafat on a five-point "bid for joint action."

The hard-liners, among them some of Arafat's closest aides, have since demanded "amendments" to the Hussein-Arafat agreement. These included a unified Arab delegation instead of just a Jordanian-Palestinian one as foreseen in the Hussein-Arafat agreement, and application of Palestinian self-determination before, rather than after, creation of a Jordanian-Palestinian confederation. The hard-liners also have reaffirmed their rejection of Resolution 242 because it refers to Palestinians only as a "refugee problem."

Hussein has been pressing Arafat to announce publicly his acceptance of the resolution in order to involve the U.S. more directly. But his accord with Arafat made no specific mention of it, referring only to holding new negotiations on the basis of unnamed U.N. Middle East resolutions.

During his appearance, Murphy presented the administration request for $815 million in economic support for Egypt, the same as this fiscal year and far below the $1.2 billion Cairo is seeking.

The administration's program summary made no mention of Israel because, State Department officials said later, no decision has been reached on Israel's request for $1.9 billion in fiscal 1986. They expressed doubt that this would be done before Israeli Finance Minister Yitzhak Modai visits Washington starting Wednesday.