Story Highlights• NEW: Clashes intensify as Lebanese forces go into camp's northern outskirts
• Islamic militants are barricaded inside refugee camp north of Tripoli
• Artillery fire some of the heaviest since the start of the campaign May 20
• Move comes a day after officials confirm second round of U.S. supply flights
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TRIPOLI, Lebanon (CNN) -- Lebanese soldiers -- backed by gunboats, heavy artillery and rows of tanks -- entered a Palestinian refugee camp that has hosted days of fierce fighting north of Tripoli, a military source said Friday.
Two Lebanese soldiers were killed Friday during clashes with Islamic militants at the Nahr el-Bared camp, the army said.
Columns of tanks rolled nearby as Lebanese troops entered the northern edges of the refugee camp, breaking more than a week of relative calm in the fighting between the army and Fatah al-Islam, a militant group alleged to be linked to al Qaeda.
Troops also pounded the camp with 155 mm shells in what is the heaviest daytime artillery fire seen since the beginning of the campaign. Lebanese security sources said the militants were pinned down in one area.
The violence began May 20 when Lebanese internal security forces were conducting raids in a Tripoli neighborhood following a bank robbery. The raids triggered clashes near the refugee camp after, army sources say, Fatah al-Islam militants shot at the troops, who returned fire.
The battles are the worst internal violence since the end of Lebanon's civil war in 1990.
The Lebanese military said it is pinpointing and targeting militant strongholds so as to avoid civilian casualties. The army has accused the militants of using civilians as human shields.
More than 30,000 Palestinians called the overcrowded camp home before fighting erupted last month. Many fled the camp last week, and Red Cross officials estimated Friday that about 10,000 civilians may still be inside.
Many Palestinians fled on foot, carrying babies and belongings, while others crowded into cars and vans and waved white flags from the windows, pleading with militants and soldiers not to shoot.
About 30 soldiers, 50 militants and one civilian have been killed, Lebanese officials said. However, Palestinian sources reported a lower death toll for militants -- 17 to 25 -- and a higher one for civilians, 20.
Friday's fighting comes a day after tank and artillery ammunition arrived in Beirut as part of a second wave of U.S. military supply flights.
Last week, the Lebanese government requested emergency assistance, saying it needed ammunition because it had depleted its stock during the operation.
Arab nations also have assisted in supplying the Lebanese military.
Prime Minister Fouad Siniora has said he refuses to "surrender to the terrorism of the militants" and accused Fatah al-Islam of using Palestinian refugees as a prop for the group's attempt to destabilize the country.
Friday's offensive defies warnings from Hezbollah leader Sheikh Hassan Nasrallah, who during a televised address last week told Lebanese forces not to enter the camp. Nasrallah also warned against sacrificing the Palestinian civilians in the camp. (Full story)
"Suppose that the government is having a war on terror, that doesn't mean they should kill half of the people on the street," he said.
The Lebanese military stays out of the Palestinian camps under a 1969 agreement that allows the Palestinians to run them.
Although critical of the Lebanese military, Nasrallah also lambasted Fatah al-Islam, accusing the group not only of an aggression against the army, but also of an aggression against all of Lebanon.
Hezbollah, an Iranian-backed Shiite faction, views extremist Sunni groups such as al Qaeda and Fatah al-Islam as enemies.
CNN's Brent Sadler contributed to this report.
Lebanese forces secure the area Friday at a refugee camp north of Tripoli where militants have been holed up.