Black smoke billowed from the Nahr al-Bared camp, home to 40,000 Palestinians, as tanks shelled positions held by Fatah al-Islam fighters hitting back with machinegun and grenade fire.
Thousands of Palestinians had fled their homes on the edges of Nahr al-Bared, where fighting was most intense, to shelter together deeper inside the coastal camp in north Lebanon, Palestinian sources inside the camp said.
Fierce fighting stopped UN and Red Cross workers from delivering essential supplies. More than 150 people had been wounded and dozens of homes destroyed, the sources said.
Abu Salim, a spokesman for Fatah al-Islam, threatened to ignite violence elsewhere if the army did not ease its bombardment. ''If the situation stays like this we will not be silent and will definitely move the battle outside (the nearby city) of Tripoli,'' he told Reuters by telephone.
He said five fighters of the group were killed and nine wounded inside the camp since fighting erupted early yesterday.
Twenty-nine soldiers have been killed and 55 wounded.
Palestinian sources in the camp said the bombardment today had killed 12 civilians.
The conflict is Lebanon's worst internal violence since the 1975-90 civil war. Sunday's battles in Nahr al-Bared and Tripoli killed 15 militants and 15 civilians.
The United States, which firmly backs the Beirut government, said Lebanon was justified in attacking the militants. The German EU presidency said it was ''deeply concerned'' at the violence and reiterated its support for the government.
Fatah al-Islam, a Sunni Muslim group which emerged late last year, has only a few hundred fighters and scant political support in Lebanon. Based in Nahr al-Bared, it is thought to have links with jihadist factions in other Palestinian camps.
A security source said the Lebanese security forces had detained 20 members of the group, including Saudis, Algerians, and a Tunisian.
The violence showed how fragile security remains in Lebanon, racked by political and sectarian tensions since last year's Israeli-Hezbollah war in the south and by a series of unsolved assassinations before and after Syria's 2005 troop pullout.
The cabinet, itself embroiled in a long-running political crisis, discussed the fighting today. Rival Lebanese factions have condemned the attacks on the army, which is carefully constructed to reflect the country's mosaic of sects.
Palestine Liberation Organisation representative Abbas Zaki said after talks with Prime Minister Fouad Siniora that the camps housing 400,000 refugees, a legacy of the Arab-Israeli war of 1948, should not be ''the spark that starts a civil war''.
SYRIAN HAND? Lebanese government ministers say Fatah al-Islam is a tool used by Syria to stir instability in an effort to derail UN moves to set up an international court to try suspects in the 2005 killing of former Prime Minister Rafik al-Hariri.
Syrian Foreign Minister Walid Moualem reiterated that his country opposed Fatah al-Islam and wanted to arrest its leaders.
''Our forces have been after them, even through Interpol,'' he said in a lecture at Damascus University. ''We reject this organisation. It does not serve the Palestinian cause and it is not after liberating Palestine.'' Under a 1969 Arab accord, Lebanon's army may not enter the refugee camps, leaving a security vacuum filled by Palestinian factions. They have not obeyed a 2004 UN Security Council resolution calling for all militias in Lebanon to be disarmed.
Hezbollah, Lebanon's biggest armed group, whose Shi'ite fighters are backed by Iran and Syria, also rejects that demand.
Fatah al-Islam's leader, Shaker al-Abssi, was sentenced to death in Jordan in absentia for the 2002 killing of a US diplomat. Abu Musab al-Zarqawi, the slain chief of al Qaeda in Iraq, received a death sentence for the same crime.
Abssi, a Palestinian guerrilla in his 50s, was jailed in Syria and fled to Lebanon after he was released last year.
Palestinian guerrillas established bases in Lebanon in the late 1960s and took part in the civil war that erupted in 1975.
REUTERS KK PM0054