MANILA, July 14 (Reuters) - President Gloria Macapagal Arroyo vowed on Saturday to seek justice for 14 marines killed in a rebel ambush in the southern Philippines this week, but promised an equal "determination to forge peace".

"We will not allow those who committed barbaric acts to hide under the negotiating table. Only principled warriors deserve a seat on it," Arroyo said in a statement.

"We will run after those who killed our marines, but we will not run away from the peace talks. Our desire to see the killers punished is matched only by our determination to forge peace."

A tense ceasefire is holding on the southern island of Basilan as the military and the country's largest Muslim rebel group, the Moro Islamic Liberation Front (MILF), separately investigate heavy fighting earlier in the week in which at least 18 people were killed and 16 were wounded.

The military says the Marines were ambushed by hundreds of armed men and 10 of those killed were beheaded. Rebels say troops entered one of their camps without permission, but added they will investigate the beheadings.

The ceasefire between the military and the MILF, which has been in place since 2003, has been occasionally broken, but Tuesday's fighting was one of the most serious violations.

"I believe we're close to an agreement and the gaps are getting narrower," Arroyo told Reuters in an interview at the Malacanang Palace on Saturday, hours before she comforted families of the Marines at an airbase in Manila.

"The encounter will not deter us in our mission. Our mission is to bring lasting peace in Mindanao and I believe this is achievable."

On Friday, a fresh battalion of marines was moved to Basilan as newspapers said the military was preparing for punitive action against those believed to have carried out the beheadings.

The marines on Basilan were checking reports that Giancarlo Bossi, an Italian Roman Catholic missionary kidnapped on the mainland last month, might have been taken there by his captors.

There are about 3,000 troops on Basilan. As well as the MILF, the island is home to the Abu Sayyaf, a smaller but deadlier rebel group blamed for the worst terrorist attack in the mainly Roman Catholic country, the bombing of a ferry that killed more than 100 people in 2004.

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