Nine other marines were wounded and at least four rebels were killed during the incident on Basilan island.

The clash took place as the marines were searching for an Italian priest.

The priest was seized in June by kidnappers believed to be from either Abu Sayyaf or the Moro Islamic Liberation Front.

Both groups are seeking an independent Islamic state in the southern Philippines.

Ceasefire violation?

The priest, named as Giancarlo Bossi, was seized by armed men near the coastal village of Bulawan near Zamboanga on 10 June.

Father Bossi was seized by armed men on his way to church

Lt Col Caculitan said that 14 marines had been killed. The number of rebels who died is unconfirmed, but reports vary from four to at least 20.

Initially, the military assumed that the rebels were mainly from the Abu Sayyaf group - the smallest but deadliest of several Islamic groups fighting the Philippine government.

But army officers say that Abu Sayyaf gunmen do not have a presence in the area where Rev Bossi was kidnapped.

A spokesman for the Moro Islamic Liberation Front (MILF) later confirmed that some its members had been involved in the clash, despite the fact the MILF is currently engaged in peace talks with the government.

Mohagher Iqbal, the chief negotiator for the MILF, said the marines had attacked one of the group's strongholds, violating a ceasefire agreement, and that his forces had therefore fought back.

"The incident could have been avoided if the government had co-ordinated with our forces in Basilan before they actually moved in," he told reporters.

But he denied that the MILF was responsible for the beheadings, or that it was involved in the kidnapping of the priest.