WASHINGTON/MANILA (Reuters) - About 1,750 American troops including elite units will go to the Philippines for joint ground operations with local forces in a stepped-up bid to destroy Muslim rebels, U.S. defense officials said on Friday.

In a major strategic shift from training to fighting, the U.S. officials said some 350 elite Special Operations soldiers well be among the American troops sent to the southern Philippines to help subdue Muslim separatist groups that have been fighting the mainly Roman Catholic nation for three decades.

The 350 Special Operations troops would head for troubled Jolo Island in the Sulu archipelago and be supported by another 400 troops stationed at Zamboanga. Meanwhile about 1,000 U.S. Marines from the 31st Marine Expeditionary Unit would remain offshore on ships to provide emergency back-up and air support.

Complaining of "leaks" concerning the shift in U.S. strategy, the Philippines refused comment on Friday on whether the Americans would fight side-by-side with their forces.

The country's constitution bars foreign troops from any combat role, but officials in Washington signaled on Thursday that U.S. soldiers would be given a new and far broader mandate to help Philippine units in their fight against Muslim guerrillas. "In this effort, the U.S. soldiers will work side-by-side in an offensive effort with the Philippine military," one Pentagon official said, speaking on condition of anonymity. Asked whether U.S. ground forces would be involved in combat, the official said: "They are going to conduct joint operations to disrupt and defeat the Abu Sayyaf group."

Washington has blacklisted Abu Sayyaf, mainly a kidnapping gang that claims to fight for an Islamic state, as a terrorist organization with links to Osama bin Laden's al Qaeda network.

Bombings by Muslim rebels and battles with troops have killed at least 200,000 people, mostly civilians, in the last 30 years. But even if Manila intends to find a way round the constitutional ban to allow U.S. troops to fight Abu Sayyaf and other groups such as the Moro Islamic Liberation Front (MILF), the move could snap back in the government's face. Leftists are already up in arms over the presence of U.S. troops for the training exercises and a bilateral military resupply pact signed in November. Many Filipinos fear that the close ties will drag the country into Washington's wars.

Pentagon officials said the American troops would be under U.S. command and an advance military assessment team would begin laying the groundwork for the operation. The training exercises due to start in Mindanao on Monday will take U.S. troops to the heartland of the Abu Sayyaf and the MILF, the largest Muslim rebel group in the Philippines.

Last year, U.S. troops accompanied Philippine units into the field to hunt for Abu Sayyaf rebels but stopped short of fighting, hanging back in makeshift camps behind the front lines.