It is unclear who the gunmen were, but the incident took place in the area where the Muslim rebel group, the Abu Sayyaf, is active.

On 7 June two hostages - including an American missionary - were killed during a rescue mission, and a third freed from the Abu Sayyaf after a year in captivity.

Philippines officials have speculated that the Abu Sayyaf might now be looking for more hostages.

Barge adrift

The four Indonesians were snatched from a tug towing a coal-laden barge from Indonesia to the central Philippine city of Cebu.

The incident happened off Basilan island, where the Abu Sayyaf have their base.

The captain and three crew - the chief officer, second officer and chief engineer - were abducted, and the barge cut adrift near the island of Tamuk, just west of Basilan.

There were 10 crew in all, but it is not clear why only four were captured.

The six left behind said eight men in military fatigues had approached the barge on a speedboat and forced their four colleagues into their boat, according to Sthillainathan Thillai, a manager for the tugboat's Singaporean owner.

The manager said there had been no ransom demands from the hijackers.

The barge was listed as missing late on Monday, and later found by Philippine Navy vessels.

US troops

On Monday, US troops on Basilan training the Philippine army to crush the Abu Sayyaf came under fire for the first time.

A spokesman for the Pentagon told AFP news agency that two US Marines who were guarding a construction site on the outskirts of Basilan's capital were attacked and returned fire.

The identity of the attackers is not clear, but they are suspected to belong to the Abu Sayyaf, which the US links to Osama Bin Laden's al-Qaeda movement.

The group says it is fighting for an independent Muslim state in the southern Philippines, but it is better known for kidnapping for ransom.

On 7 June Filipina nurse Ediborah Yap and US Christian missionary Martin Burnham were shot dead in a failed rescue mission by the Philippines military.

Mr Burnham's wife, Gracia, was injured but was freed and has since returned to the US.