The soldiers, who were traveling aboard trucks with militiamen, fired back at the attackers, who fled into a nearby forest, dragging some wounded rebels. Police and troops sent as reinforcements were pursuing the guerrillas, Lanao del Sur police Chief Paniares Adap said.
The soldiers were on their way to distribute funds to militiamen at a number of outposts when they came under attack from dozens of Muslim guerrillas, army officials said.
Rebel spokesman Eid Kabalu confirmed that the attackers were members of the 11,000-strong rebel organization but said he was checking what triggered the attack.
"They won't attack without any justifiable ground," Kabalu told The Associated Press by telephone, adding that the group maintains many strongholds in mountainous Lanao del Sur, about 500 miles southeast of Manila.
The guerrilla attack came four days after the government ended a massive three-day assault that dislodged about 1,000 rebels from 15 predominantly Christian farming villages they had seized in southern North Cotabato province, displacing about 80,000 people.
Despite the rebel pullout from North Cotabato, a predominantly Christian province of more than a million people, military chief Gen. Alexander Yano said last week that security in the southern Mindanao region remained "volatile and fluid."
Yano said the rebels could be bracing to launch attacks in other areas of Mindanao. The sprawling southern region, homeland of minority Muslims in the predominantly Roman Catholic Philippines, has been the scene of a bloody Muslim separatist insurrection that erupted in the early 1970s.
Philippine officials suspect the Muslim guerrillas detonated three bombs, all fashioned from mortar rounds, that damaged two power transmission pylons late Wednesday and wounded a suspected Muslim rebel Thursday in two North Cotabato areas. Three other unexploded bombs were found in the province late last week, police said.
The North Cotabato conflict, which the military said killed two soldiers and up to 31 rebels, erupted at a crucial juncture in peace negotiations between the government and the rebels.
The two sides, which have signed a 2003 cease-fire, have reached agreement on the size of a future expanded Muslim homeland. But the signing of the accord was halted two weeks ago by the Supreme Court, which acted on a petition filed by Christian politicians wary of losing land and power to Muslims.
The Supreme Court opened a hearing under tight security Friday to determine whether the accord violated the country's 1987 constitution. The issue has divided many Filipinos and sparked protests in recent weeks by the contending camps.