A UN aid convoy which entered the Nahr al-Bared camp in northern Lebanon has been forced to leave after shells exploded near its vehicles.

Fighting has resumed between Islamist militants besieged by the Lebanese army in the Palestinian refugee camp despite a ceasefire declared by the militants.

Six lorries went into the camp during a lull, carrying food, water, medical supplies and an electricity generator.

There are reports of injuries and it is not clear if the aid was delivered.

About 31,000 civilians are trapped in the camp in deteriorating conditions. Dozens have been killed since the violence erupted on Sunday.

An eyewitness told the BBC he had seen the dead and wounded lying in the streets.

The head of a hospital in a nearby Palestinian refugee camp, Dr Youssef Asaad, told the BBC that ambulances had been able to take 13 wounded civilians to hospitals outside the camp.

He appealed to the Lebanese army and the militants to give medical organizations the chance to evacuate more of the injured.

'Open-ended' truce

The Lebanese army and members of the militant group Fatah al-Islam, who are inside the camp, have been exchanging heavy fire for a third day.

The BBC's Jon Leyne in Beirut says there was a brief ceasefire but the fighting never completely stopped and now it is back to full force.

Speaking earlier on Tuesday Fatah al-Islam spokesman Abu Salim said his group was willing to abide by a truce from 1430 (1130 GMT).

"We are giving a chance for calm and a ceasefire... It is open-ended if the army commits to it as well," he said.

The Lebanese military said it would not commit to a formal ceasefire, but reiterated that its forces would not be the first to open fire.

"We only return fire when we are fired upon. If there is no firing at us, we will not return fire," a military source said.

On Monday evening, Lebanon's Cabinet authorized the army to step up its efforts and "end the terrorist phenomenon that is alien to the values and nature of the Palestinian people".

Syrian denial

Overnight, US President George W Bush accused the Islamists of "trying to topple the young democracy" and said they needed to be "reined in".

Lebanese Trade Minister Sami Haddad told the BBC his government suspected Syria of masterminding the violence in an attempt to destabilize the government.

Syria's ambassador to the UN, Bashar Jaafari, has denied his country has any link to the group, and said some of them had been in jail in Syria for their support for al-Qaeda.

The European Union's foreign policy chief, Javier Solana, is meeting Lebanese Prime Minister Fouad Siniora in Beirut to discuss the fighting - the bloodiest internal conflict in Lebanon since the civil war ended 17 years ago.

The clashes erupted when security forces tried to arrest suspects in a bank robbery. Militants from Fatah al-Islam, a radical Palestinian splinter group, then attacked army posts at the entrances to the camp.

Lebanon is home to more than 350,000 Palestinian refugees many of whom fled or left their homes when Israel was created in 1948.

PHOTO CAPTION

Smoke coming from the Nahr al-Bared refugee camp