The United States has accepted Israel's contention that the planned partial withdrawal of some of its forces in Lebanon is only a first stage in the removal of all its troops from that country, Secretary of State George P. Shultz said yesterday.

"Any notion that this redeployment is part of an effort to stay in Lebanon is totally wrong," Shultz told reporters at the end of three days of talks with the visiting Israeli ministers of defense and foreign affairs. "Exactly to the contrary, it is a step in the right direction."

Foreign Minister Yitzhak Shamir and Defense Minister Moshe Arens spent a total of 15 hours convincing Shultz and other senior administration officials that the deployment of some of Israel's 30,000 troops to the south of the Awali River would not lead to the de facto partition of Lebanon.

The United States was initally reluctant to accept Israel's unilateral decision on the partial pullback, but reportedly decided that, since the Israelis were determined to go ahead, the wisest course was to seek the maximum possible coordination with the Israeli government.

"This is a decision that has been taken by Israel and we are all determined to ensure that it contributes to our shared goals for Lebanon," said a senior administration official, briefing reporters yesterday.

He also acknowledged that the Israeli pullback bears no direct relation to the implementation of the May 17 agreement between Israel and Lebanon on the withdrawal of all foreign forces from Lebanon.

"Obviously," the official said, "we've got a problem. The agreement stipulated the timing. The Israelis have made clear, however, that their guarantee of total withdrawal according to the timetable of the agreement was related directly to Syrian and PLO willingness to withdraw simultaneously. We obviously at this point are not talking about fulfilling those provisions."

The official denied, however, that the United States had tried to pressure Israel into setting a date for a further staged pullback in Lebanon.

"We were not convincing the Israelis that their redeployment had to be put in a broader context," he said. "They know that and they want to be as helpful as they can."

Having accepted the Israeli pullback as a fait accompli, the president is now dispatching his new special envoy, Robert C. McFarlane, on a first regional tour to coordinate moves between Israel and Lebanon and also to try to persuade the Syrian government of President Hafez al-Assad to join the troop withdrawal process.

McFarlane leaves today on a tour that is expected to take him to Syria, Lebanon, Israel, Jordan, Egypt and Saudi Arabia. Reagan last week appointed him to replace Philip C. Habib.

Lebanese President Amin Gemayel said during a visit to Washington last week that he feared the planned Israeli pullback, from Beirut, the Beirut-Damascus highway and the Shouf Mountains, would lead to the de facto partition of his country between Israel and Syria and hamper attempts to strengthen the authority of his fledgling government.

The briefing official said that Reagan, who met for 30 minutes yesterday with the Israeli ministers, was prepared to consider expanding the role of the four-nation multinational peace-keeping force in Lebanon, which includes a 1,200-man U.S. Marine contingent, on the condition that an agreement is first reached between the Gemayel government and Druse groups in the Shouf Mountains.

"The Israelis well understand that if the Lebanese government is successfully going to increase its control over this area there will have to be such an agreement," the official said.

But Arens said later: "Our withdrawal is not contingent upon anything but our own security interests." He added however that he believed there was a "good chance" of an accommodation being reached in the Shouf. The Israelis told Shultz they were prepared to use their "good offices" to help secure such an agreement.

Arens acknowledged that the challenge facing the Lebanese army was "not a simple one." But, he said, "we are hopeful that with assistance from all sides they will be able to handle the situation."

In his meeting with the Israeli ministers, Reagan reaffirmed his commitment to a Middle East peace based on U.N. Resolution 242 of 1967, the Camp David process and his own Sept. 1, 1982, initiative that has sought, so far in vain, to draw Jordan into talks about the future of the Palestinians in the Israeli-occupied West Bank and Gaza Strip.

Shamir returned to Israel yesterday. Arens is staying on for talks with Defense Secretary Caspar W. Weinberger about military aid and strategic cooperation.