Gloria Arroyo initially downplayed claims the ferry was bombed

President Gloria Arroyo, who initially downplayed claims of responsibility from the Abu Sayyaf group, has admitted that militants were behind the attack.

She said six members of Abu Sayyaf had been charged with murder and attempted murder over the attack in February.

Two of the six were arrested in March, but the other four - including two of the group's leaders - remain at large.

The 10,000 ton Superferry 14 was heading for Bacolod in the central Philippines when it caught fire, on 27 February 2004.

According to officials, 116 of the 900 people onboard are now presumed to have died, although only 63 bodies have been recovered.

The report into the fire has now concluded it was caused by "an explosive device," national police spokesman Superintendent Leopoldo Bataoil said in a statement.

Investigators believe the Superferry was targeted because its owners, WG&A, refused a request for $1m in protection money from Abu Sayyaf in 2003 .

The Abu Sayyaf group claimed responsibility soon after the incident, but the government initially dismissed the claim as a "propaganda ploy".

In March, however, a group of Abu Sayyaf members were arrested over the incident - including one man who allegedly confessed to planting the bomb.

On Monday Mrs Arroyo said six people had been charged over the incident - adding that the two already in custody had also been found responsible for the abduction of a group of tourists from the island of Palawan in 2001.

The kidnapping eventually resulted in the deaths of several hostages, including two Americans.

Mrs Arroyo called for police to step up their efforts to find the other four men charged over the ferry bombing.

"I am now instructing the police and the military to intensify the manhunt for the two masterminds - Khaddafy Janjalani and Abu Sulaiman - and their two other accomplices," she said on Monday.

Sulaiman and Janjalani, thought to be Abu Sayyaf's main leader, already have $5m bounties on their heads.

Abu Sayyaf - which mainly engages in kidnap for ransom - is one of four Muslim rebel groups operating in the southern Philippines.

It has been labelled a terrorist organisation by both Manila and Washington, and is believed by the US to have links with Osama Bin Laden's al-Qaeda network.

The Philippine Government has deployed thousands of troops in the south in an effort to eradicate the Abu Sayyaf.