The first U.S. soldiers to arrive with assault rifles strapped to their backs flew into the southern Philippines on Thursday to help prepare for a joint military exercise aimed at fighting a Muslim extremist group.

The 13 troops arrived on a massive U.S. Air Force C-17 transport plane, toting unloaded M-16s. They sweated heavily under the tropical sun on the tarmac of Edwin Andrews Air Base in the Zamboanga area.

They are "logistics soldiers here to support the training between the Philippine and the U.S. soldiers," said U.S. Lt. Col. Steve Woods, spokesman for the exercise.

Another 10 soldiers without visible weapons flew in later in the day on a C-130 transport plane that also carried several crates of equipment. Fourteen other Americans had arrived Friday.

Thursday's arrivals brought to 65 the number of U.S. troops in the Zamboanga area for a six-month mission to train Filipino soldiers to fight the Abu Sayyaf, a Muslim rebel group that has been linked to Usama bin Laden's Al Qaeda network.

The Abu Sayyaf, notorious for kidnappings and beheadings, is holding an American missionary couple and Filipino nurse hostage on Basilan island, close to Zamboanga, home to the Philippine military's Southern Command.

More than 600 U.S. troops, including 16 from the Special Forces, are to take part in the mission.

Presidential spokesman Rigoberto Tiglao said Wednesday the exercise is aimed at upgrading the Philippine military's counter-terrorist capability. The first month would entail preparation of a "curriculum," followed by four months of actual training and one month of evaluation, he said.

News that some U.S. troops will visit combat zones and carry weapons has sparked some local opposition to the American presence. The Philippine Constitution bars foreign soldiers from fighting on sovereign soil.

Seeking to ease opposition, Lt. Gen. Roy Cimatu, head of the southern Philippine forces, said the U.S. soldiers will have almost "no probability" of being in combat.

"I will see to it that they will not be going on any combat patrols. That should dispel rumors that they will be in combat operations," Cimatu said.

In Zamboanga, U.S. soldiers would be forbidden from going out at night and would be escorted every time they leave military bases, he said.

The Philippine Senate began televised hearings Thursday on the constitutionality of the maneuvers. National Security Adviser Roilo Golez said Wednesday they are permitted by the bilateral Visiting Forces Agreement.

Several senators worried about the possible escalation of U.S. involvement in the campaign against the Abu Sayyaf despite repeated assurances that U.S. troops would not be allowed to engage in combat.

"The operations to react to any particular situation will be conducted by Philippine forces under a Philippine commander," Defense Secretary Angelo Reyes said. "Even if 10 Americans get killed ... they have no command and control, no authority to conduct operation."

Golez said only 160 U.S. troops would be deployed on Basilan, mostly in teams of 12 to each of the eight combat battalions involved in trying to rescue American missionaries Martin and Gracia Burnham of Wichita, Kan., and Filipino nurse Deborah Yap.

He said two U.S. soldiers will deployed to one Philippine army or marine company, comprising 120 soldiers, while six others would stay at battalion headquarters. Three companies comprise one battalion.

"They will be to the rear of the Filipino troops, so that would really avoid the possibility of them being cornered or being put in a situation where they have to defend themselves," Golez said.

A presidential human rights committee would be set up on Basilan to receive any complaints.

Golez said in addition to the 160 U.S. troops on Basilan, 250 will be stationed in Zamboanga, and 250 mainly support and aircraft maintenance crew in Cebu.