suspected positions of the Fatah Islam militants, seeking to destroy the group with al-Qaida ties or force them out of the Nahr el-Bared camp on the outskirts of this northern port city, Lebanon's second largest. A plume of black smoke billowed from an unknown target in the camp.

The army brought in reinforcements from other regions. Two trucks towing field artillery were seen heading toward Tripoli on the coastal highway late Monday.

The renewed fighting ended an overnight lull amid efforts for an informal cease-fire between the two sides. It was not known what sparked the exchanges.

Palestinian factions attempted to broker a cease-fire. The representative of the Palestinian militant group Islamic Jihad, Abu Ahmed Rifai, said Fatah Islam militants pledged to cease firing and withdraw from positions facing Lebanese troops. A senior officer at Lebanese army command would not say a cease-fire was reached but repeated the military's stance that it will not shoot if it does not come under fire.

At least 27 soldiers and 20 militants were killed Sunday. Two more soldiers were killed Monday night when a mortar fired from inside camp struck their vehicle.

On Monday, troops blasted the camp with artillery and tank fire, smashing buildings and sending plumes of black smoke towering over the crowded camp on the Mediterranean.

The fierce battle that began Sunday also has killed an unknown number of civilians, raising fears that Lebanon's worst internal violence since the 1975-1990 civil war could spread in a country with an uneasy balancing act among various sects and factions.

Fighting paused briefly Monday afternoon to allow the evacuation of 18 wounded civilians, according to Saleh Badran, an official with the Palestinian Red Crescent Society. But the fierce fighting quickly resumed.

Palestinian refugees hid in their homes as fighting raged, and Palestinian officials in the camp said nine civilians were killed Monday. Reports from the camp could not be confirmed because officials and reporters could not get inside.

"There are many wounded. We're under siege. There is a shortage of bread, medicine and electricity. There are children under the rubble" of damaged buildings, Sana Abu Faraj, a resident of the camp, told Al-Jazeera television by cell phone Monday.

The camp is more like a small town, with more than 31,000 people living in two- or three-story white buildings on densely packed narrow streets alongside mosques, schools and businesses.

It is one of more than 12 impoverished camps that are home to more than 215,000 refugees, out of a total of 400,000 Palestinians here.

Hundreds of Lebanese troops surrounded Nahr el-Bared, staying outside in accordance with a nearly 40-year-old agreement with the Palestinians. The troops pounded the camp with artillery and tank fire, and militants responded with gunfire and mortar rounds.

The army is seeking to uproot Fatah Islam, which arose in the camp late last year.

Major Palestinian factions have distanced themselves from Fatah Islam, which touts itself as a Palestinian liberation movement. But many view it as a nascent branch of al-Qaida-style terrorism with ambitions of carrying out attacks around the region.

Nevertheless, the military assault adds yet another layer of instability to Lebanon's potentially explosive politics. Prime Minister Fuad Saniora's government already faces a domestic political crisis, with the opposition led by Iranian- and Syrian-backed Hezbollah demanding its removal.

Raising fears of spreading violence, an explosion went off in a shopping area in a Sunni Muslim sector of Beirut later Monday, wrecking parked cars and injuring five people a day after a bomb blast in a Christian part of the capital killed a woman. Although there were no claims of responsibility, the confluence of two bombings in as many days while the fighting was going on in Tripoli was highly unusual.

Saniora now risks sparking a backlash among Palestinians in Lebanon's other refugee camps, where armed groups and Islamic extremists have been growing in influence and, in at least one case, have been sending recruits to fight U.S. troops in Iraq. If the military moves into Nahr el-Bared in force, it could trigger widespread anger around the Arab world, particularly at a time when Israel is battling Palestinians in the Gaza Strip.

A spokesman for Fatah Islam, Abu Salim, warned that if the army siege did not stop, the militants would step up attacks by rockets and artillery "and would take the battle outside Tripoli."

"It is a life-or-death battle. Their aim is to wipe out Fatah Islam. We will respond and we know how to respond," he told The Associated Press from the camp.

The White House said it supports Saniora's efforts to deal with the fighting, and the State Department defended the Lebanese army, saying it was working in a "legitimate manner" against "provocations by violent extremists" operating in the camp.

The leader of Fatah Islam, Palestinian Shaker al-Absi, has been linked to the former head of al-Qaida in Iraq and is accused in the 2002 assassination of a U.S. diplomat in Jordan. He moved into Nahr el-Bared last fall after being expelled from Syria, where he was in custody.

Since then, he is believed to have recruited about 100 fighters, including militants from Saudi Arabia, Yemen and other Arab countries, and he has said he follows the ideology of al-Qaida leader Osama bin Laden. Among the militants killed in the fighting Sunday was a man suspected in a plot to bomb trains in Germany last year, according to Lebanese security officials.

Lebanese security officials accuse Syria of backing Fatah Islam as a tool to disrupt the country.

Syria controlled Lebanon until 2005 when its troops were forced to withdraw from the country following the assassination of former Lebanese prime minister Rafik Hariri.

Syrian Foreign Minister Walid Moallem insisted Monday Damascus had nothing to do with the Fatah Islam. "Fatah Islam is rejected and does not serve the Palestinian cause. On the contrary, it harms it in every way," he said.