Syrian-backed leftist fighters charged into the outskirts of Tripoli, in northern Lebanon, yesterday in an offensive against strongholds of the fundamentalist Tawheed militia but were stalled by tenacious fighting.

Syrian mediators announced Friday night that they were pulling out of negotiations in the face of intransigence by Saeed Chaaban, leader of Tawheed, a radical sunni Moslem movement that seeks to bring an Iranian-type government to Lebanon.

Shortly before dawn, a barrage of artillery and rockets thundered into Tripoli from Syrian-controlled hills, followed by forces of the pro-Syrian Baath Party, the National Syrian Socialist Party and the Lebanese Communist Party, special correspondent Nora Boustany reported.

As they advanced on Tripoli, the leftist fighters joined the Syrian-sponsored Arab Democratic Party, which has been battling Chaaban's radical Sunni Moslem coalition for the past two weeks in an effort to disarm it under terms of a year-old Syrian-negotiated agreement.

About 800 Chaaban followers, brandishing modern weapons and swords, blocked the leftist attackers from the center of the city and its port.

It was impossible to get reliable casualty reports, but International Red Cross officials said the morgues of Tripoli's hospitals were filled.

In Beirut, meanwhile, at least 27 persons were killed and 50 wounded in shelling that hit Christian and Moslem residential areas. The Lebanese Forces, the combined Christian militias, accused the Shiite Moslem Amal militia of provoking the fighting, and Amal accused the Lebanese Army.

The Lebanese Forces and the Army suggested that the escalation was aimed at thwarting Syrian efforts to bring the warring factions together.