President Reagan's special Middle East envoy, Philip C. Habib, and other U.S. ambassadors in the region will meet here next week to discuss strategy for helping to implement the Israel-Lebanon peace agreement, a senior State Department official said yesterday.

Nicholas A. Veliotes, assistant secretary for Mideast affairs, told a House subcommittee that the main focus will be on ways of inducing Syria to cooperate with the effort to withdraw all foreign forces from Lebanon. Israel has said it will not pull out its forces unless there is a simultaneous withdrawal by the 50,000 Syrian troops and by the Palestine Liberation Organization forces in Lebanon.

Testifying before the subcommittee on European and Middle East affairs, Veliotes conceded that the administration might have been "too optimistic" in believing that Syrian cooperation could be obtained speedily in the wake of the Israeli-Lebanese accord negotiated by Secretary of State George P. Shultz.

"But the game is not over," Veliotes said as he described American efforts to convince Syria, both directly and through friendly Arab states, that its security interests will be served best by withdrawal.

He noted the high state of tension existing between Syrian and Israeli forces in Lebanon and said: "My concern is that there will be a miscalculation that will lead to hostilities."

"We will be reassessing our role next week," he said, referring to the planned meeting. "Then the secretary will be consulting the president about how to move the process forward."

After his testimony, Veliotes explained to reporters that his use of the term "reassessing" did not imply any shifts in U.S. policy. The purpose of the meeting, he said, was to take advantage of the fact that Habib, several other American ambassadors and Lebanese Foreign Minister Elie Salem will be here at the same time and to use the opportunity to discuss possible steps.

Veliotes also insisted that Saudi Arabia, which gives substantial financial aid to Syria, is working quietly behind the scenes to prepare the way for a Lebanese-Syrian agreement. He added, "I wouldn't be surprised" if the Saudis' "profile is somewhat raised in the coming weeks."

Subcommittee Chairman Lee H. Hamilton (D-Ind.) noted the assessment of the Marine commander in Lebanon that the Marines assigned to the multinational force there probably will have to remain for another 15 months.

"I certainly don't quarrel with it," Veliotes replied.