The Philippine military accused Muslim rebels Wednesday of killing 19 soldiers on a remote southern island in one of the worst outbreaks of violence between the two sides in years.
The fighting further complicated efforts to end one of Asia's longest insurgencies, with the Moro Islamic Liberation Front and the military trading accusations of breaking a ceasefire in place to promote peace talks.
The military said troops were on the hunt for members of the Al Qaeda-linked Abu Sayyaf group in the jungles of Basilan island when MILF rebels ambushed them, triggering a nine-hour clash.
Nineteen soldiers died, 13 were wounded and three remained missing, regional military commander Lieutenant General Raymundo Ferrer told reporters.
"We shall not only file ceasefire violations against them, but also murder chargers against their members who are responsible for killing and wounding our soldiers," Ferrer said.
The military initially reported that 13 soldiers had been killed in the fighting, and that six troops were taken hostage.
But the bodies of the six soldiers were found on Wednesday, prompting allegations from the military that they had been killed while in captivity.
"They were definitely murdered... they were were still alive last night," army spokesman Antonio Parlade said in Manila.
The 12,000-strong MILF has waged a rebellion since the 1970s for an independent Islamic state or autonomous-rule in the southern third of the mainly Catholic Philippines.
The rebellion has left about 150,000 people dead, with most of the deaths occurring in the 1970s when an all-out war raged.
Although the two sides signed a truce in 2003 that paved the way for peace talks, the ceasefire is frequently marred by clashes across the vast southern Mindanao region that Muslims claim as their ancestral homeland.
The talks also broke down in August when the MILF rejected the government's roadmap for peace, heightening tensions in Mindanao.
Tuesday's fighting was the most deadly between the MILF and the military since 2008, when two rogue rebels launched a series of attacks mostly on Christian towns elsewhere in Mindanao that left about 400 people dead.
It was also the worst on Basilan island since 2007, when 14 Marines were beheaded after a gunbattle with MILF rebels.
Regional military spokesman Lieutenant Colonel Randolph Cabangbang said the MILF commander accused of the 2007 atrocity, Dan Laksaw Ansawi, also led Tuesday's MILF assault.
The military often accuses the MILF of providing shelter to the Abu Sayyaf, a small band of militants believed to number fewer than 400 but which is blamed for the country's worst terrorist attacks.
However the MILF denied it was responsible for Tuesday's fighting.
MILF spokesman Von al Haq said the violence erupted after the soldiers strayed into rebel territory.
He also said five rebels had died in the clashes, while the military claimed nine guerrillas were killed.
Basilan, a remote and small island with a mostly Muslim community, has long been a stronghold of the Abu Sayyaf because it can rely on the support of locals.
About 600 people living near the town where Tuesday's fighting fled their homes in fear of further violence, civil defence chief Benito Ramos said in Manila.
However both the Philippine military and the MILF said they would seek to follow ceasefire procedures, and there were no further clashes on Wednesday.
The government also reaffirmed its commitment to the peace process, expressing hope that negotiations could resume next month.