US troops have come under fire in the Philippines for the first time since they began joint operations to crush Abu Sayyaf rebels.

About 10 suspected rebels fired briefly on US military engineers working on a road project in Maligue on the southern island of Basilan.

US Marines guarding them returned fire, resulting in an unknown number of casualties.

Security has now been stepped up, but the US said similar infrastructure projects being undertaken by the US troops would not be affected.

More than 1,000 US troops are in the southern Philippines helping local forces in hunting the Abu Sayyaf, who have been linked to the al-Qaeda network of Osama bin Laden.

Some 340 of them are US engineers, known as the Seabees, who are building roads, wells and rebuilding an abandoned airstrip in Basilan.

Hostage killed

A number of Abu Sayyaf rebels are known to be in the island's dense jungle.

Elite US Navy Seals form part of the US force

The Americans are barred from taking part in combat except in self-defence.

But defence department spokesman Melchor Rosales said the US Marines had acted within the guidelines that governed the joint operations.

US Navy construction teams are also building bridges, ports and helicopter landing zones to help the Philippine military move around the island and foster goodwill among the island's 300,000 residents.

Two weeks ago Filipino forces recovered US hostage Gracia Burnham in a bloody rescue in which her husband, Martin, and a Filipina nurse were killed.