The New York Times, citing Malaysian state media, said villagers beat to death a man suspected of being one of the foreign invaders, bringing the death toll to 21 on the idyllic Pacific island and tourist destination.
The fighters, who appear to have come from an island about an hour away by speedboat in the Philippines, claim to be heirs of the sultanate Sulu, who once ruled northern Borneo for centuries. The Times said multiple members of the group claim to be the present sultan and disagreement exists over whether they should have attempted to reclaim the territory.
Malaysian Prime Minister Najib Razak said he has doubled the forces deployed to the area to end the conflict, the Times reported.
"Let's give them the opportunity and time to carry out their operations and overpower the group and rescue those in need," he said.
He said tourist resorts on the island remain open and villagers and tourists alike should not fear for their safety.
Officials from neighboring Philippines sent a diplomatic envoy to its close ally to help smooth tensions and offer assistance in the conflict.