The Reagan administration put Israel on notice publicly today that it still expects the imminent partial withdrawal of Israeli troops in Lebanon to be only the first phase of a complete pullback.

The White House issued a statement saying that "we are confident" that Israel does not intend to participate in a permanent partition of Lebanon and noting previous Israeli promises to withdraw completely.

The statement, read by presidential spokesman Larry Speakes to reporters here, followed reports from the Mideast that the Israelis are preparing for a long stay when they pull back to new lines at the Awwali River north of Sidon.

"In recent weeks some have questioned the motives underlying this withdrawal by Israel, saying that it presages a permanent partition of Lebanon," Speakes said. "We are convinced that the Israeli government is committed to the full withdrawal of its forces in Lebanon."

Israeli Prime Minister Menachem Begin's government has said several times that Israel has no territorial ambitions in Lebanon and that its forces will leave once Israel's northern borders are secure from terrorist attack.

However, the Israeli-Lebanese peace agreement worked out in May with the help of Secretary of State George P. Shultz specifies that Israeli withdrawal is contingent upon a simultaneous pullout of Syrian and Palestine Liberation Organization forces from Lebanon, and that point is spelled out in a separate U.S.-Israeli memorandum of understanding accompanying the Lebanon accord.

According to another administration official, who asked not to be identified, the White House statement was made out of concern that the Israeli pullback could lead to new factional strife in the Chouf Mountains area, southeast of Beirut, that Israel intends to evacuate.

The official said the United States is hoping that Israel and Lebanon will "cooperate" in making the withdrawal to help the Lebanese government take control of the area.

"A coordinated effort, and not just Israel pulling up and leaving, would be advantageous," he said.

This view was reflected in today's White House statement, which said that, while the United States will encourage early withdrawal by Israeli forces, "the governments of Israel and Lebanon will coordinate the smooth and orderly return of responsibility" in the Alayh-Chouf areas.

The partial Israeli pullback, which officials have said will begin in a matter of days, is designed to reduce casualties and the cost of maintaining the Israeli army in Lebanon.

It will involve the evacuation of about 370 square miles of territory held by Israel since its invasion a year ago, including the southern outskirts of Beirut, the Beirut-to-Damascus highway and the Chouf Mountains, scene of recent heavy fighting between Christian and Druze militias.

The White House statement today reported on "several conclusions" said to have been reached by Middle East special envoys Robert C. McFarlane and Richard Fairbanks after extensive travel in the region in recent weeks.

The "first and foremost" conclusion, the statement said, is the "solid foundation" of support in the Arab world and Israel for the "principles of full withdrawal and full authority for the Lebanese government."

But it also noted the "enormity of the task" facing the Lebanese government in attempting to "strengthen the consensus" among the warring factions in Lebanon.

Ending the strife among these groups is "essential to the reconstruction and revitalization of the Lebanese economy and the establishment of political stability," the White House statement said. "It is clear that President Gemayel is committed to these goals."

The statement said that the Lebanese government "will soon face an opportunity" to restore stability and "extend the process of reconstruction" in the Alayh and Chouf districts that Israel intends to evacuate. Israel has said it intends to remain at the Awwali River until Syria and the Palestine Liberation Organization agree to withdraw from northern and eastern Lebanon. Israel has announced no timetable for complete withdrawal and the U.S. "knows of none," Speakes said.

"I cannot give you any report of progress on Syrian or PLO withdrawal," he added.

The administration official said today's statement was issued by the White House, and not the State Department, because "we want to make it very clear" that President Reagan "understands Israel is committed to full withdrawal."