Two US soldiers and a Filipino marine have been killed in a landmine blast on the southern Philippine island of Jolo, a Philippine army spokesman has said.
Two other Filipino soldiers were also wounded in the explosion, which hit their vehicle near the town of Indanan.
The Philippines' military said last week that it had captured Indanan, a stronghold of Abu Sayyaf rebels.
The US has about 300 soldiers in the southern Philippines, advising the local army in fighting insurgents.
The American soldiers are the first to be killed in the Philippines since 2002, when one serviceman died in an bombing in the port city of Zamboanga, also in the south of the country.
Under a US agreement with the Philippines its troops are not allowed to take part in combat unless attacked. Otherwise, they are there to train and advise the Philippine army in counter-insurgency operations.
"We're still investigating to determine who was behind the explosion," Philippine army spokesman Lt-Col Romeo Brawner said in the capital, Manila.
The Philippines army said it captured Indanan town last week
He said the improvised landmine exploded near a Philippine marine outpost in Indanan.
The town, in the hilly interior of Jolo, has been a stronghold of the Muslim insurgent group Abu Sayyaf.
The army said last week that it had captured the town after heavy fighting.
Abu Sayyaf is one of the smallest but deadliest Islamist militant groups in the largely Roman Catholic Philippines.
Established in the early 1990s, it has kidnapped dozens of foreign aid workers, missionaries and tourists in the south and was blamed for the country's worst terrorist strike, the bombing of a ferry in 2004 that killed more than 100 people.