DAMASCUS, SYRIA, OCT. 11 -- Lebanon's Syrian-backed President Elias Hrawi has asked Damascus to help oust rival Christian leader Gen. Michel Aoun from his besieged stronghold, and there were reports tonight that thousands of Syrian troops, backed by tanks and artillery, were moving toward Lebanon's Christian enclave.

Travelers coming from Shtawrah in central Lebanon said they saw hundreds of tanks and Syrian troops moving down the main Beirut-Damascus highway toward the Lebanese capital tonight in what appeared to be preparations for a final showdown with the defiant Aoun.

Witnesses and security sources told the Reuter news service that Syrian special units and infantry, supported by tanks and artillery, moved forward in Beirut and in strategic mountainous areas overlooking Aoun's enclave. Residents saw tanks rolling through the southern suburbs and deployed less than 700 yards from Aoun's lines.

{In Washington, a State Department official cast doubt on the reports, saying, "Our people here don't know of any large number of Syrian troops that have suddenly been inserted into Beirut," and speculated that any seen might be part of a crackdown on Iranian-backed Hezbollah militia forces, Reuter reported.}

The Syrian move could add a new element of instability and strife to a Middle East region already in turmoil because of the Persian Gulf crisis and the clash in Jerusalem this week in which 19 Palestinians were killed by Israeli police.

It also could signal the beginning of a new battle for control of Lebanon, in which Damascus seeks to extend its authority throughout the country. Sources here said Hrawi's request had given Syria legitimate authority to intervene militarily in Lebanon.

Hrawi's request for Syrian help, reportedly delivered in Damascus on Wednesday, follows the ratification by Lebanon's parliament and Hrawi's signing of a new constitutional document that was negotiated in Taif, Saudi Arabia, last year. The Taif accords, which received final endorsement from Hrawi on Sept. 21, would end the old power-sharing formula that gave dominance to Lebanon's Maronite Christian minority and would instead strike an equal balance between Moslems and Christians in the parliament.

The Taif accords also include a clause that allows the Lebanese government to ask for Syrian backing to extend its authority over all Lebanese territory.

The United States has supported the Taif accords and Syria's role in maintaining order in the country. Edward Djerejian, the U.S. ambassador to Damascus, said in an interview today: "U.S. policy toward Syria is based on the premise of identifying common interests where they exist and, if there is common ground, to build on them like we have in Lebanon and the gulf crisis.

"In Lebanon, our objective is implementation of the Taif accords," he said.

Aoun has blocked implementation of the accords, calling them a capitulation to Syrian domination of Lebanon. From his bombed-out presidential palace in the Christian enclave, he has resisted the agreement, which would commit Syria, at least on paper, to a pullback from Beirut and surrounding areas to the Bekaa Valley within two years.

A two-week-old food and fuel blockade imposed against the Christian heartland, which is mainly under Aoun's control, does not appear to have weakened his resolve.

Syria made no official statement about the request of the Lebanese government, which has been struggling for nearly a year to consolidate its power while Aoun has rallied Christian hard-liners to his side. Damascus has often reiterated its desire to crush Aoun but has so far been restrained by reservations in the West, mainly in France, about a military assault on Christian areas.

Sources here said Syria has recently supplied Gen. Emile Lahoud's loyalist Lebanese army force of 10,000 with armored personnel carriers and tanks. Aoun's army of 15,000, previously funded and armed by Iraq, has also been the recipient of major contributions from Lebanese emigres moved by his nationalistic rhetoric.

Fierce battles that broke out last January between the Christian militia of Samir Geagea and Aoun's soldiers shattered Christian solidarity against Syria and drove thousands of Christian civilians into Moslem areas. Most Christian legislators who took part in drafting the Taif accords were barred from returning to their homes and have lived in plush hotels in the mainly Moslem sector of Beirut with financial subsidies from Saudi Arabia.

Aoun has insisted that he is the only legitimate head of government, despite the election of Hrawi by legislators meeting in Syrian-controlled territory late last year. Aoun was appointed prime minister in September 1988 to prepare for presidential elections when the parliament could not agree on a replacement for president Amin Gemayel, who left office then.