Secretary of State Alexander M. Haig Jr. said yesterday that the United States is "very concerned" about the latest Israeli-Palestinian clashes acrossthe Lebanese border, and the State Department said later the President Reagan's special Lebanon mediator, Phillip C. Habib, will come here later this week to discuss the situation.

Haig's comment on the latest disruptions of the Lebanon cease-fire came in answer to questions from reporters as he appeared at the department's daily news briefing to announce some shifts in key State Department personnel.

Howeverr, department spokesman Dean Fischer said later that the United States is encouraged by statements from the Israeli government and the Palestine Liberation Organization that they want to preserve the shaky cease-fire arranged under Habib's mediation last summer.

Other department sources said Habib, who has been on a private visit to China, probably will arrive here Thursday. That will be too late for him to meet with Haig, who is to leave Wednesday on a trip to Europe, but the sources said Habib probably will talk with Reagan.

These sources also said it is unlikely that Habib will go out to the Middle East at this time, especially if the situation along the Israeli-Lebanese border appears to be stabilizing. However, they added, a final decision will depend on whether the fighting has stopped and whether it appears that a trip by Habib would serve a useful purpose in shoring up the cease-fire.

On Sunday, Israeli planes attacked PLO positions in Lebanon for the second time in three weeks, and Palestinian forces responded with heavy rocket fire into Israel. The situation is regarded as worrisome by U.S. officials who fear an escalation of fighting that could lead Israeli ground forces to invade Lebanon in an effort to knock out PLO positions in the southern part of that country.

That, in turn, could deal a serious setback to the administration's hopes of getting the stalled Egyptian-Israeli negotiations moving in a way that finally will produce a breakthrough toward an agreement on autonomy for the Palestinian inhabitants of the Israeli-occupied West Bank and Gaza Strip.

Haig's special assistant for the autonomy talks, Richard Fairbanks, is in the Middle East trying to pave the way for an early, high-level negotiating session. However, the effort is being hampered by Begin's insistence that the meeting take place in Jerusalem, which is unacceptable to Egyptian President Hosni Mubarak because he refuses to recognize Jerusalem as the capital of the Jewish state.

In his personnel announcement, Haig confirmed that Richard R. Burt, who has been the department's director of politico-military affairs, will become assistant secretary for European affairs, and that James L. Buckley will switch from undersecretary for security affairs to become counselor of the department.

Haig also announced that Rear Adm. Jonathan Howe, senior military assistant to Deputy Defense Secretary Frank Carlucci, will succeed Burt.

Although the appointments had been expected, they had been delayed for a considerable time, apparently because of opposition from Haig's opponents in the White House and among hard-line Republican conservatives in Congress. The secretary's unusual gesture in coming to a routine news briefing to announce them personally appeared to be an attempt to underscore that he had won out over his critics in the naming of key aides.