Syrian troops have been engaged in heavy fighting in a week-long clash with Moslem fundamentalists in the central Syrian city of Hamah, according to State Department sources and foreign reports.

The State Department reported yesterday that the Army had sealed off the city, long a stronghold of resistance to the government of President Hafez Assad. No casualty figures were confirmed, but a State Department official, who asked not to be identified, said the fighting "appears to be a major, serious clash."

In a dispatch from Damascus, The Associated Press reported that Western diplomatic sources there estimated that 6,000 to 8,000 troops had ringed the city and that 360 soldiers and "many hundreds" of rebels had been killed. The sources said about 100 government tanks and artillery pieces, supported by aerial attacks-- possibly rockets fired from helicopters--launched a constant barrage on the city.

Hospitals in Damascus and two other Syrian cities were said to be filled with casualties from the fighting, in which antigovernment forces used rocket-propelled grenades and other heavy weapons. The casualty figures could not be confirmed independently.

A State Department official said most of the fighting appeared to be confined to the city of more than 100,000, located 120 miles north of Damascus, Syria's capital. Skirmishes first began 10 days ago and escalated into a full-scale clash on Feb. 5, according to the official, who stressed that the State Department's information was preliminary and incomplete.

The Syrian government issued a statement last night in which it branded the State Department's information "pure lies." While the statement conceded that some violence had occurred in Hamah, it called "the situation inside Syria . . . normal and strong," The AP reported.

The clash appears to be the strongest challenge to date to Assad's 12-year rule of Syria, which has been subjected to monthly terrorist bombings and harsh government reprisals in recent months. Diplomatic sources in Amman said last week the government had decided on an all-out crackdown on fundamentalist opposition both within the Syrian Army and in Hamah.

The military operation follows by about three weeks the arrest of at least 18 Army officers and the roundup of hundreds of dissidents after what Western intelligence sources said was an abortive military coup. It is unclear whether there is a direct relationship between the reported abortive coup and the violence in Hamah, however.

It also was unclear yesterday how the new violence would affect Syria's regional interests. Syria maintains more than 20,000 troops as an Arab peace-keeping force in neighboring Lebanon and has hostile relations not only with archenemy Israel, but also with Iraq and Jordan, its Arab neighbors to the east and south, both of which it has accused of aiding dissident groups inside the country.

The fighting in Hamah appears to be the most violent of a long series of confrontations between the Army and residents of the city, considered a center of Syria's Sunni Moslem majority. It is also said to be a headquarters for cells of the underground Moslem Brotherhood, a Sunni group pledged to overthrow Assad's government, which is dominated by the country's Alawite minority.

A harsh, year-long crackdown by a special Army unit and Syrian secret police that included the reported massacre of at least 200 of the city's residents appears to have failed to break the back of Sunni resistance.

There were conflicting reports about the origins of the new clash. Agence France-Presse reported from Bonn yesterday that rebel leaders, in a communique, had claimed credit for staging an uprising that had seized the city. But diplomatic sources in Amman said as long as a week ago that the Syrian government had long planned the military operation and had surrounded the city at least a week ago in preparation for a sweep of the town.

The diplomats also said they believed a military coup had been brewing but were uncertain how far it had advanced before the government uncovered it and began making arrests in mid-January. Reports on the number of arrests of Army officers vary from a Syrian government figure of 18 to several hundred. There have been unconfirmed reports that between 30 and 50 officers have been executed.

Sources said the trouble within the Army, unrelated to the confrontation at Hamah, began in the southern border town of Dara, over an attempted purge of suspected dissidents. Fighting broke out in which several soldiers were killed.