Lt. Col. Romeo Brawner, a military spokesman, said a joint convoy joined was on its way to an area in Jolo's Indanan township when the land mine exploded.
The convoy was assisting an operation to build two schools and dig wells in the township.
"They were not in combat," Brawner told reporters. "These US soldiers were there in the area to supervise the developmental projects in Indanan."
Brawner said no suspects had been immediately identified, but suspicion immediately fell on the Abu Sayyaf.
The group, which has been linked to al Qaeda, is blamed for numerous bombings, beheadings and kidnappings of Filipinos and foreigners in the south in recent years.