MANILA, Philippines - Two US Navy soldiers and one Filipino marine were killed on Tuesday in a land mine attack in a stronghold of Islamic militants in the southern Philippines, officials said.

Philippine army spokesman Lieutenant-Colonel Romeo Brawner said two other Filipino soldiers were also wounded when the vehicle they were riding in struck an improvised land mine near Indanan town on the island of Jolo.

They were the first deaths since 2002 among US forces deployed in troubled areas in the southern Philippines. In that year, one soldier in a restaurant was killed by a bomb on a motorcycle parked outside.

"We're still investigating to determine who was behind the explosion," Brawner told reporters, adding the latest attack happened near a Marine outpost in Indanan.

He said the two US Seabees were in a convoy of vehicles returning to base after inspecting humanitarian projects in the area when the land mine detonated.

"They were not on combat operations," he told reporters. "They were part of a US Navy construction unit. They were non-combatants helping poor Muslim communities on Jolo."

The US embassy in Manila issued a statement confirming the death of two servicemen, but withheld their identities pending notification of the next of kin.

"They lost their lives serving others, and we will always be grateful for their contributions to improve the quality of life on Jolo," US Ambassador Kristie Kenny said in a statement.

The US servicemen, escorted by Philippine troops, were on a re-supply mission for a school construction project on the island, the statement said.

Since 2002, the United States has deployed about 300 soldiers to help train, advise and assist local troops fight a small group of Abu Sayyaf rebels, who have close ties to the regional militant network, Jemaah Islamiah (JI).

Based on an agreement with Manila, US troops are not allowed to participate in actual combat, but are authorized to fight back when attacked. Rebels have fired at US soldiers on Jolo island in past incidents.

Last month, the Philippine military lost 23 soldiers during an assault on the Abu Sayyaf's main base near Indanan, one of the biggest battles since 2007.