U.S. troops, in possible violation of the Philippines' constitution, have taken part in combat operations against guerrillas linked to Al Qaeda, an activist group said in a report Monday.

U.S. Embassy spokesman Matthew Lussenhop disputed the allegation.

"Visiting U.S. troops in the Philippines advise, assist, share information with their Philippine counterparts, but they do not engage in combat and they have no direct role in combat operations. Any combat operations are 100 percent Filipino," he told The Associated Press.

The group Focus on the Global South, citing a number of U.S. military writings, doctrines and eyewitness accounts to back its claim, said an independent investigation should be conducted to determine whether the alleged combat operations violated the Philippine constitution.

The U.S. Special Forces contingent has been deployed in the south since 2002, nearly a year after the Muslim extremist group Abu Sayyaf kidnapped three Americans and 17 Filipinos from a resort. One of the Americans was beheaded soon after the kidnapping and another was killed during a military rescue operation the following year.

Lussenhop said the number of Special Forces troops average "no more than a few hundred at any one time."

Herbert Docena, who wrote the report for the activist group that promotes human rights and fair trade, said U.S. troops may also be violating the Philippine constitutional ban on the stationing of foreign military bases in the country since the they have not left the south since they arrived five years ago.

Lussenhop, however, said the facilities and camps being used by the Americans were only temporary.

The U.S. military presence in this former American colony is a sensitive issue, heightened following the recent conviction of a U.S. Marine on rape charges. The Marine was not part of the U.S. task force.

Docena said eyewitnesses claim to have seen U.S. troops near hostilities, operating military equipment, defusing land mines and performing other war-related activities. U.S. troops also operate spy planes over areas of hostilities, including one unmanned aircraft that crashed last year, he said.

Philippine Senator Rodolfo Biazon, head of the Senate defense committee, said U.S. troops are explicitly banned from joining Filipino troops on combat patrols or operations.

U.S. forces, however, are allowed to fire back if they come under attack, he said.

Meanwhile, the Philippines' largest Muslim rebel group on Monday protested several members' arrests related to bombings last Wednesday. They warned that the arrests could hurt efforts to end two decades of separatist rebellion.

Police said they filed an illegal possession of explosives complaint against four suspects, including two identified by the Moro Islamic Liberation Front, or MILF, as its members.

The blasts in the southern cities of General Santos, Kidapawan and Cotabato killed seven people and wounded 44 others.

Philippine security officials have accused the MILF of having ties with the Al Qaeda-linked Southeast Asian terror network Jemaah Islamiyah, but Muslim rebel leaders have denied such links.