The Lebanese army continued their offensive against the Fatah al-Islam militants, who are holing up in the Palestinian refugee camp of Nahr al-Bared in northern Lebanon, with heavy pounding throughout Sunday.

Meanwhile, the army clashed with Islamic fighters at another Palestinian refugee camp, raising the specter of a possible spreading of violence to other parts of the country.

The fresh outbreak of violence occurred at the Ein el-Helweh camp at the suburb of southern city of Sidon.

The fighting started as militants from the Syria-based Jund al-Sham group threw explosives at an army position at the edge of the camp. The attack sparked fierce exchanges of rifle fire and grenades between Lebanese soldiers and Jund al-Sham members.

The fighting also involved the mainstream Fatah organization at the army's side, according to local media. The mainstream Fatah group backed the army in its fighting against the Fatah al-Islam militants, which it said "had nothing to do with the Palestinians nor Islam."

According to medical sources in Sidon, so far five people have been injured in the fighting.

Ein el-Helweh is the largest of the 12 Palestinian refugee camps in Lebanon. Protests had been launched there days after the outbreak of deadly fighting between the army and Fatah al-Islam militants in the Nahr al-Bared camp in northern Lebanon on May 20.

In the north, the military, backed by tanks and helicopters, has tightened its siege around the camp where Fatah al-Islam militiamen are still holding out.

Earlier, Lebanon's national news agency NNA reported that four Islamist militants were killed in Nahr al-Bared camp on Sunday, and two soldiers, who had been killed overnight, were buried on Sunday.

According to the pan-Arab al-Jazeera TV, more than 16 people, fighters and civilians, have died in the camp over the weekend. Fatah al-Islam said it has lost three fighters. But the precise death toll inside the camp is not known.

Fatah al-Islam over the weekend refused renewed demands by the army and Prime Minister Fouad Seniora to surrender. A spokesman said that they would rather die than give up their weapons.

The militants in Nahr al-Bared had also called on other radical groups to join them in their battle with the army, after fighting escalated on Friday.