A senior Lebanese government source said today that there was little doubt Syria could bring about the release of seven Americans and five other westerners being held captive by Moslem radicals here and that he expected Syria would obtain their release "at the right moment."

The statement, by a Lebanese official who spoke on condition that he not be identified, came as Lebanese Moslem leaders announced a wide-ranging plan to improve security, with Syrian assistance, at Beirut International Airport and in Moslem west Beirut.

The security plan was announced today by Lebanese Prime Minister Rashid Karami after long talks in Damascus yesterday between Syrian officials and Moslem and Druze leaders from Lebanon.

It calls for creation of a coordinating committee that would group Syrian observers, members of the dominant Druze and Shiite Moslem militias in the Moslem-controlled part of Beirut, and representatives to be appointed by the defense and prime ministers.

The communique from the Damascus talks also deplored a campaign by the Reagan administration to arrange an international boycott of the Beirut airport -- focal point of the Trans World Airlines hijacking last month. The communique said this effort showed "the malice of American ruling quarters" and was intended to punish Lebanon for abrogating a troop-withdrawal and security agreement reached with Israel in 1983.

Security at the airport and an end to fighting between Moslem and Druze militias in west Beirut are seen here as the necessary first steps toward restoring peace in Lebanon, torn by 10 years of civil war,, Palestinian-Moslem fighting and violence by radical Moslem factions -- and an Israeli invasion.

The Syrian-engineered security plan aims broadly at pacification of Beirut and broad political reforms, but the initial reaction of some Lebanese Christian leaders was skepticism that it would have any effect.

Today was the first time any senior Lebanese government figure had spoken openly about the fate of the missing kidnaping victims -- who include seven Americans, some of them in captivity for more than a year.

The senior Lebanese official, stressing his conviction that Syria could obtain their release, said: "Syria could really deliver them immediately, but there are some limitations on Syria. I am sure if Syria put all its weight on the kidnapers it could locate these individuals, but it may require far more involvement than Syria believes is proper at the moment.

"I think the Syrian role at present is to ensure that these individuals are not harmed and, at the right moment, to exercise its influence to make sure they are released."

Syria was instrumental in the release of the 39 American hostages in the TWA hijacking last month, including four who were held separately by a radical Moslem faction, and this has led Damascus to try to capitalize on its influence for U.S. recognition as a regional power.

The Lebanese source said it was not Syrian policy to have people kidnaped or hijacked, but he conceded that Syria "has given in the past lots of elbow room to parties and groupings operating in Lebanon. It is part of Syrian political tactics to permit autonomy of action to certain of its allies, always with some contours that could be broadly defined or broadly ill-defined."

He added that some people working under the Syrian umbrella had "gone to extremes" -- an indirect reference to Hezbollah, a militant Shiite Moslem group that has links to Iran and is based in the Syrian-controlled Bekaa Valley in Lebanon. Hezbollah is believed by U.S. officials to have engineered the TWA hijacking.

Most observers here believe that to free the seven Americans Syria would have to take military action against the Shiite extremists holding them. The Islamic Jihad organization, which says it is holding the Americans and two Frenchmen, has made their release contingent on the freeing of Shiite militants imprisoned by Kuwait for a series of fatal bombings at U.S., French and Kuwaiti facilities in December 1983. Kuwait has said it would not bow to pressure.

Syria, in the view of observers here, is waiting for some kind of signal from Washington before it decides that a further gesture toward the United States is timely. Syrian President Hafez Assad said in a speech last month that he would do his best to secure the release of the missing Americans.

The American captives are William Buckley, a U.S. Embassy political officer; the Rev. Benjamin Weir, a Presbyterian minister; the Rev. Lawrence Jenco, a Roman Catholic priest; Terry A. Anderson, The Associated Press bureau chief; and three officials of the American University of Beirut -- Thomas Sutherland, Peter Kilburn and David Jacobsen.

The security plan for west Beirut and the airport was drawn up during talks in Damascus yesterday under the chairmanship of Syrian Vice President Abdul Halim Khaddam.

The communique, issued at dawn in the Syrian capital, called for a new Lebanese constitution and political reforms that would ensure wider representation for Lebanon's Moslems.

In the only public Lebanese government comment on the Damascus talks today, President Amin Gemayel told foreign journalists he could not yet evaluate their outcome.

The senior Lebanese government source emphasized what he called Syria's pivotal role and interest in stabilizing Lebanon.

"Syria has nothing but great glory to reap from success in Lebanon," he said. "If it fails in Lebanon, whatever happens in Lebanon will repeat itself in Syria."

The source stressed the need for a continued Syrian presence here, saying the Lebanese government "has no one to turn to but Damascus. The effective force in Lebanon is Syria."

The source, however, expressed reservations on the inclusion of officials from the warring factions in the security coordinating committee, saying that similar attempts in the past had failed.

The statement issued in Syria rejected the notion of self-security in the embattled Palestinian refugee camps and outlined a role for the Lebanese police force for collecting arms and the disbanding militias with the help of a special mixed Lebanese Army force.

The government source said a special Army unit of 5,000 to 10,000 men would be formed from the various Moslem and Christian brigades and would be provided with the necessary backup from Syria.

Following a Syrian-Lebanese summit conference in late May, Gemayel declared that the 30,000 Syrian troops already deployed in central and northern Lebanon would give the Lebanese Army support when the need arises in implementing security plans in and around Beirut.

Security conditions in Tripoli, in the north, Lebanon's second-largest city, deteriorated sharply during the past two days with the Sunni Moslem militia of the Islamic Unification Movement battling with Syrian-supported militiamen. The fighting erupted after the Sunni force, which is close to Yasser Arafat, criticized the Damascus talks.

In west Beirut, hit-and-run attacks against Shiite soldiers of the Lebanese Army continue nightly.

Two sons of former Christian Phalangist deputy Edmond Rizk were kidnaped in west Beirut and four members of a Sunni Moslem family were shot to death, police said.

The Damascus meeting, which grouped 13 key Sunni, Shiite and Druze leaders and religious figures, was aimed primarily at defusing tensions between the various communities. The Sunni and Druze minorities are uneasy with the growing power of Justice Minister Nabih Berri's Shiite Amal movement. Clashes break out routinely between the Druze Progressive Socialist Party, which now includes many frustrated Sunni Moslem militants, and Amal.

The senior government source said the envisioned Lebanese Army task force had to have Syria's political cover to provide it with the aura of authority to influence and intimidate warring factions having connections to Damascus. Christian leaders were skeptical of the latest salvation formula, with many saying that unless results were felt soon they saw no solution "for what is going on in Lebanon."

Former president Camille Chamoun, a Christian, complained that the planned security measures in west Beirut were "an example of self-security and political autonomy" that skirted the Cabinet -- which has not met for months -- and the Army command.