President Reagan firmly told Syrian President Hafez Assad this week that the United States expected his help in winning the release of seven Americans still held captive in Lebanon, administration sources said yesterday.

Assad reacted negatively to Reagan's call and "didn't like being told" by the United States what he was expected to do, the sources added.

One official said that Reagan delivered to Assad a lot of "straight talk" about the need to release the other seven hostages, but that the Syrian president was not receptive.

Assad played a crucial role in winning the release of 39 Americans held hostage in Beirut after the hijacking of Trans World Airlines Flight 847.

Earlier this week, administration officials said they believed that Assad would work "in earnest" to free the remaining seven hostages held by Islamic extremist groups in Lebanon.

But the Syrian president's cool response to Reagan could indicate that he does not intend to be as cooperative in winning release of the seven as he was in gaining freedom for the 39 captives who came home this week.

The seven, some of whom have been held captive since early last year, are believed to be in the hands of radical Shiite Moslem groups associated with the Hezbollah, or Party of God, which also carried out the hijacking of TWA Flight 847 on June 14.

Assad's intervention was apparently key to the release of four Americans held by Hezbollah after a snag developed in the effort to free the 39 TWA passengers last weekend.

Assad also proposed the diplomatic formulation that resolved the conflict between the hijackers' demands for release of Lebanese prisoners held in Israel and the U.S. refusal to make concessions to terrorists.

A high-ranking Israeli official said this week that he believes that Syra has the power to free the seven Americans, who are believed to be held in Lebanon's Bekaa Valley, which is heavily dominated by Syria.

David Kimche, director general of Israel's Foreign Ministry, suggested that Syrian intelligence probably knows where the Americans are located.

But, he added, "The fact that they haven't done anything about them, I think, says more about Syria's sincerity to help than anything else."

State Department officials have said that they think Assad knows, or has known, the whereabouts of the seven hostages, but that to free them without the cooperation of their captors would require military action that could risk death to the captives.

The White House disclosed Tuesday that Reagan and Assad had talked Monday for about 15 minutes.

Officials said Reagan had thanked Assad and asked that he use his influence to gain release of the remaining hostages, but the strained nature of the exchange had not been disclosed.

Also Monday, aides to Assad in Damascus expressed "displeasure" with the Reagan administration's perceived lack of gratitude for Assad's crucial role in release of the 39 Americans. "We do not want to beg for gratitude," one official said.

In his nationally televised Oval Office address Sunday after the 39 Americans left Damascus, Reagan mentioned that "Sryia has had a central responsibility" in their release.

But Reagan also mentioned Algeria, Saudi Arabia and King Hussein of Jordan in his address, delivered soon after the hostages were airborne.

Syria has displayed hostility toward efforts by King Hussein to revive Arab-Israeli peace talks.

National security affairs adviser Robert C. McFarlane said in a television interview this week that there remains "considerable disagreement" between Washington and Damascus over Jordan's efforts to revive the peace process.

Syria has also been accused by Jordan and the United States of fomenting terrorism in the Middle East. State Department spokesman Bernard Kalb said this week that Syria remains on the U.S. list of nations supporting terrorism. Kalb said that "according to reliable reports, a number of terrorist organizations have received some form of support from Syria . . . or have training facilities there or in parts of Lebanon under Syrian control."

A White House official said last week that administration analysts believe that Assad's chief motivation in helping free the 39 Americans was to strengthen his hand in Lebanon and to increase stability among rival factions there.

Despite differences with Assad, the Nixon, Ford and Carter administrations recognized Syria as a key factor in the Middle East.

The Reagan administration, however, has given a cold shoulder to Assad, beginning with the first Middle East trip by then-Secretary of State Alexander M. Haig and continuing through deepening conflicts over Lebanon under Secretary of State George P. Shultz.