Fierce fighting for control of the strategic central city of Zahle continued today, and combat zones near the Israeli border and in Beirut and its southeastern suburbs also flared perilously.

The second announced cease-fire in 24 hours in Zahle, 30 miles east of Beirut, collapsed, the state-run Beirut radio said. Zahle residents, contacted by phone, said there were times when the shelling slackened but that it had never stopped.

In the south, after the port cities of Sidon and Tyre had been shelled this morning, clashes between Israeli-backed Christian militias in the six-mile Lebanese border strips and Palestinian-leftist guerrillas intensified, security sources reported, knocking out power lines in many villages.

In Beirut, mortar shells slammed deep into the mainly Christian east and predominantly Moslem west of the capital as tension along the dividing Green Line increased.

The duels appeared to involve the Palestine Liberation Army, which is the Palestinian component of the Syrian Army, and leftist Lebanese groups on one side, and the regular Lebanese Army in the eastern part of town.

A Defense Ministry statement late tonight said, "For the fourth consecutive day, the Lebanese Army has been subjected to continuous fire" at its positions along the line dividing the hostile forces in Beirut. The Army statement said five soldiers had been wounded.

Voice of Lebanon, the private radio station of the right-wing Phalangist Party, said that shells were falling near the Baabda Palace of President Elias Sarkis.

[In Washington, the State Department said the United States is making diplomatic efforts to help "bring the fighting and human suffering to an end and to support the Lebanese government in this crisis," Reuter reported.]

Phalangist radio said it had information that there was going to be "decisive international action" "to put and end to the shelling of innocent people in Lebanon." It added that two unnamed foreign powers and placed their "resources at the disposal of the Lebanese government." This report could not be independently confirmed.

Sources said Syrian President Hafez Assad had agreed in principle to the deployment of the Lebanese Army in the besieged town of Zahle. But, the sources added, alliance of Christian militia groups under the command of Phalangist militia leader Bashir Germayel were not willing to accept a "flimsy cease-fire."

Syrian troops came to Lebanon as a peace-keeping force after the 1975-76 civil war, but now are constantly engaged in fighting Christian militias of the Phalangists. The fighting this week has been the worst in three years.

The all-Syrian Arab Deterrent Force explained that the heavy shelling of Zahle, a market town with a mainly Greek Catholic population of 160,000, was intended to deter Christian militias from building a road linking Zahle to the Christian-controlled areas to the northwest.

A political analyst said that the Syrians feared a pincer movement by Gemayel-led Christian militias from the north and Israel in the south, squeezing Syrian troops and Palestinian forces, centered south of the Bekaa Valley. Control of Zahle was vital for the Syrians, the analyst said.

In Damascus, officials said that Assad received a message from French President Valery Giscard d'Estaing, delivered by former foreign minister Louis de Guiringaud. The officials said Assad also met with Palestine Liberation Organization leader Yasser Arafat and leftist Lebanese politicians.