MANILA, May 15 (Reuters) - Fighting erupted between Philippine security forces and Muslim rebels on a troubled southern island on Thursday and both sides exchanged accusations of breaking a five-year-old ceasefire.

It is the first reported fighting since Malaysian peacekeepers started withdrawing from the region last week after peace talks stalled over constitutional issues.

Colonel Rustico Guerrero, Marine brigade commander on Basilan island, said rebels from the Moro Islamic Liberation Front (MILF) fired at a group of soldiers guarding the opening of a new road in Tipo-tipo town.

"Our troops fired back," Guerrero told reporters, adding the soldiers defended their positions. There were no reports of casualties on either side and the shooting soon ended.

The rebels said the soldiers fired the first shots when the Marines harassed a group of coconut farmers in an area known to be rebel territory.

"They started the firefight," Mohaqher Iqbal, MILF chief peace negotiator, told Reuters in a phone interview. "They have the bad habit of entering into our camps without coordination. They're provoking our fighters."

Manila has been holding talks to end a secessionist conflict with the MILF that has killed 120,000 people since the late 1960s in the south of the mainly Roman Catholic state.

But talks, brokered by Malaysia from 2001, have been stalled since December 2007 due to constitutional issues. Malaysia started to pull out its peace monitors to signal displeasure at the long delay in negotiations, but has said it will continue to broker the talks.

Residents have said the reduced presence of peace monitors could lead to ceasefire violations and fail to prevent local fighting from escalating into a full-blown conflict.

Local officials said dozens of villagers in nearly Muslim communities fled their homes and farms to avoid getting caught in the crossfire on Thursday.

A leading think-tank urged Washington on Tuesday to help Manila finalise a peace deal with the Muslim rebels to boost its counter-terrorism campaign in the region.

The International Crisis Group warned hostilities may erupt later this year if peace negotiations between Manila and the MILF to wrap a deal creating a semi-autonomous Muslim homeland in the south did not resume. (Reporting by Manny Mogato; editing by Raju Gopalakrishnan and Sanjeev Miglani)

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