Rival Shiite Moslem militias fortified their positions or brought in reinforcements Tuesday, taking advantage of a lull in fighting that came after Syria warned the four-day carnage must stop.

Police said no casualties were reported overnight from the confrontations in the slums of south Beirut.The fighting, which had simmered down to sporadic gunfire by Tuesday morning, has killed 154 people and wounded 367 since it broke out Friday. The casualty toll included 65 people killed and 50 wounded on Monday.

Police said 36 Syrian officers moved into the area with six committees in an effort to implement a truce in fighting between the pro-Syrian Amal militia and the Iranian-backed Hezbollah, or Party of God.

The battle for control of Beirut's southern slums, where an estimated 250,000 Shiites live, raised fears about the fate of 18 foreign hostages. They include nine Americans and most are believed held by Hezbollah-affiliated kidnappers in the 16-square-mile Shiite enclave.

The committees - each of which has a Syrian officer, an Iranian Embassy official, a representative of Amal and another of Hezbollah - set up truce observation posts in the slums before dawn, a police spokesman said.

The committees "are trying to convince the warring factions to observe a cease-fire as a first step," said the spokesman, who cannot be identified in line with standing regulations.

A three-point proposal announced Monday calls for a truce, withdrawal of gunmen from streets and return of the warring factions to positions they controlled before the fighting began, he said.

Hezbollah, which has gained control of half of south Beirut in the clashes, "rejects the return to pre-Friday positions," he said.

Hezbollah gunmen, the spokesman said, were using the relative lull to barricade their positions.