NAHR AL-BARED, Lebanon (Reuters) - Lebanese troops unleashed artillery and tank barrages at al Qaeda-inspired militants dug-in at a Palestinian refugee camp on Sunday, the third day of a military assault to crush the gunmen. The troops had seized and destroyed several positions of the Fatah al-Islam group at the entrances of Nahr al-Bared and were tightening their grip on them, security sources said. But the militants, who have vowed to fight to the death, were putting up stiff resistance despite three days of near constant pounding from army tanks, artillery and gunships. Explosions rocked the camp as the crackle of machinegun fire echoed. Plumes of smoke rose from the camp as shelling set buildings on fire. A mid-morning lull was shattered shortly after noon by more fierce army bombardment. A Palestinian source had said of the pause: "It could be the calm before the storm." The lull allowed relief workers to evacuate a wounded civilian from the camp, witnesses said. The shelling since Friday has devastated large parts of the camp, bringing down buildings used by the gunmen to fire at the troops but also destroying many civilian homes. "There is no square metre that has not been hit by a shell," one camp resident told Reuters by telephone earlier. "We can't leave the building we are in, let alone the street, to find out the full extent of the devastation." Most of Nahr al-Bared's nearly 40,000 population had fled to other refugee camps in the past two weeks due to increasingly desperate humanitarian conditions. A soldier was killed in overnight fighting and two wounded soldiers died, security sources said, raising to nine the number of soldiers killed since Friday. Palestinian sources said a militant commander, Naim Ghali aka Abu Riyadh, was killed by an army sniper on Saturday. More than 16 people -- militants and civilians -- have died in the camp. Fatah al-Islam said it has lost three fighters. THREAT TO PEACEKEEPERS? In what was seen as a direct threat against U.N. peacekeepers in south Lebanon, the militants' spokesman, Abu Salim Taha, told Reuters on Saturday night that a UNIFIL naval force joined the fighting, hitting a civilian shelter and inflicting casualties. A UNIFIL spokeswoman denied the peacekeepers played any role in the fighting and said the claim was "utterly unfounded". The fighting, which erupted on May 20, is Lebanon's worst internal violence since the 1975-1990 civil war. The government says militants triggered the siege by attacking army positions around the camp and Lebanon's second largest city, Tripoli. Lebanon's anti-Syrian cabinet says Fatah al-Islam is a Syrian tool, but Damascus denies any links to the group and says its leader, Shaker al-Abssi, is on Syria's wanted list. Abssi and his comrades say they are inspired by al Qaeda's ideology. Lebanon has been split by a deep seven-month-old political crisis over the opposition's demands for more say in government. The opposition includes Syria's allies, led by Hezbollah. The army began its push towards the camp on Friday with the aim of killing the militants or forcing them to surrender. Prime Minister Fouad Siniora said the militants have no choice but to surrender and give up their arms. Fatah al-Islam has vowed not to surrender or give up its weapons, saying it was ready to fight for a long time. The death toll in the two-week-old conflict stood at 110, of whom 44 are soldiers, and at least 35 are militants and 20 are civilians. While the army has not entered the camp's official boundaries, it has seized on the militants' positions on its outskirts, confining militants to about a third of the camp. A 1969 Arab agreement prevents the army from entering Lebanon's 12 Palestinian camps, home to 400,000 refugees.