Witnesses said the army had been shelling the camp since early morning, often at a rate of seven to 10 tank shells per minute.
Lebanese artillery targeted positions in the south of the camp which lies on the shores of the Mediterranean Sea in northern Lebanon while ground troops reportedly battled against Fatah al-Islam fighters in the southern and eastern sectors.
On Wednesday, the eve of the anniversary of the start of last year’s Israel-Hezbollah war, Fuad Siniora, the prime minister, called for the army to “put a final end” to the Fatah al-Islam “terrorists”.
Security and political sources said that the army had deployed extra troops in the area and was expected to use helicopter gunships and naval boats as well as tanks and heavy artillery in any assault on the camp.
“Fatah al-Islam is still fighting back,” Zeina Khodr, Al Jazeera’s correspondent outside the camp said.
“It is still difficult to know whether the army will advance into the Old Camp where the alleyways are even narrower and it will be more difficult for an advancing army to engage them.”
About 140 Palestinian fighters, not connected to the Fatah al-Islam group, were taken by military trucks to a Lebanese army barracks, a Palestine Liberation Organisation (PLO) source said.
Around 20 women, believed to be Palestinian refugees, also left the camp separately on a bus, relief workers said.
They later arrived in the nearby camp of Beddawi, which has served as shelter for the bulk of the refugees displaced by the fighting.
The camp, which has run short of fresh food and water, has been largely deserted since almost all of its population of 31,000 people fled.
Relief workers have been unable to get supplies into the camp since June 20, forcing remaining residents to scavenge for food and water in abandoned houses.
More than 170 people have been killed during the 54 days that the camp has been under siege from the Lebanese army.
The fighting erupted when Fatah al-Islam fighters launched a string of attacks on soldiers, killing 27 of them around the camp and in nearby Tripoli, according to the military.