By Manny Mogato MANILA, April 17 (Reuters) - Fighting between government forces and rogue Muslim rebels is spreading in the southern Philippines, shattering hopes for peace and threatening local support for a U.S.-backed campaign to flush out militants. A military spokesman said on Tuesday that army commandos were fanning out into the jungles of Jolo island, 600 miles (950 km) south of Manila, to hunt members of the Moro National Liberation Front (MNLF) after three days of pitched battle. "Our troops were now pursuing a separate group of MNLF rebels in another part of the island," Lieutenant-Colonel Bartolome Bacarro told reporters. Seventeen rebels, three soldiers and one civilian have been killed since renegade MNLF commander Habier Malik fired mortars at marines on Friday night, triggering fierce retaliation by the military, which dropped 250-pound bombs on his base. Nearly 8,500 families have fled the fighting and thousands crammed into schools and gymnasiums in downtown Jolo, relying on food rations from disaster agencies. The mass evacuations are a bitter development for conflict-scarred residents, who had hoped Jolo was becoming a more stable place after a long-running military campaign, backed by U.S. advisers, had rid the island of scores of militants. "I hope the two sides will come to their senses and stop this madness," Parouk Hussin, MNLF foreign relations chairman, told Reuters. "This would definitely jeopardise all our efforts to restore peace on Jolo and put to waste the gains of the peace process." LOCAL SUPPORT In its campaign to destroy the Abu Sayyaf, the most militant of four Muslim rebel groups in the largely Catholic country, the military had been careful to avoid the use of air strikes in order to win round locals, tired of so-called "friendly fire". But the troops' use of heavy bombs over the weekend and their targeting of the MNLF, which is seen as having more legitimacy than Abu Sayyaf, could undermine crucial local support. "It's going to complicate things because the MNLF probably have more local contacts, more traction with the locals then the Abu Sayyaf, who tend to be more thuggish," Tom Green, executive director of Pacific Strategies & Assessments, told Reuters. "Going against the MNLF means that a broader spectrum of people are affected because of blood ties, fathers, sons, uncles, brothers, that is going to complicate things." The Philippines has been trying to quell Muslim separatism for decades and signed a peace deal with the MNLF in 1996 that was touted as the solution to decades of conflict in the south that has killed over 120,000 people. But the deal was not implemented correctly. The Organisation of the Islamic Conference (OIC), which brokered the 1996 deal, is trying to salvage the agreement. It has called a meeting with government officials and the MNLF in Saudi Arabia in July. But both sides have ignored OIC calls to end the clashes. Armed forces chief General Hermogenes Esperon said on Tuesday that Malik, who is suspected of sheltering members of the Abu Sayyaf, could not go unpunished. "We don't want that to happen." he said. (Additional reporting by Carmel Crimmins)