Mohagher Iqbal, chief negotiator of the 11,000-strong Moro Islamic Liberation Front, said more than 50 percent of the issues being tackled by the rebels and government negotiators in Malaysian-brokered talks have been settled in principle.
President Benigno Aquino III has expressed hope that a peace pact can be signed as early as December.
Asked if he shared Aquino's optimism, Iqbal told The Associated Press that "If the current trend of negotiations will not change, I think it's highly probable."
But he added that "negotiations are unpredictable. Nothing's agreed until you sign and agree."
The Moro group has been battling for decades for self-rule for minority Muslims in the south of the predominantly Roman Catholic country. The rebellion has left more than 120,000 people dead, held back progress in the resource-rich south and sparked worries by Western governments that rebel strongholds could serve as breeding grounds for al-Qaida-linked extremists.
The planned signing of a preliminary peace pact in 2008 was scuttled when the Supreme Court declared the agreement unconstitutional, prompting three rebel commanders to launch attacks on Christian communities in the south. The attacks and an ensuing military offensive
Last month, a rebel faction opposed to the talks attacked several army camps and outposts in two southern provinces, sparking two weeks of sporadic fighting that left more than 50 dead and displaced 45,000 others. The new unrest eased after the military launched a crackdown.
Instead of undermining the talks, the recent violence prompted the government and the main rebel group "to move with more urgency," Iqbal said.
But he warned that the now-weakened breakaway faction, which he said has fewer than 200 armed fighters, could exploit any more delays or failure in the negotiations. The faction, the Bangsamoro Islamic Freedom Movement, has vowed to continue fighting for an independent Muslim homeland, arguing that the negotiations have gone nowhere.
While progress has been made in the talks, Iqbal said the two sides are still trying to fully resolve the most contentious issues, including the extent of power, revenues and territory to be granted to a Muslim-administered region in the south.