Malaysian security forces on Tuesday began attacking a group of armed men from the Philippines who had clashed with police after staking a claim to a remote part of the island of Borneo, authorities said.

The group of Filipino men, believed to number between 100 and 300, arrived three weeks ago on the east coast of the Malaysian state of Sabah, on Borneo, demanding to be recognized as representatives of a sultanate that used to rule the area.

Malaysian authorities initially tried to persuade the men to return peacefully to their homes in the nearby southern Philippines. But the intruders refused to budge, and the standoff turned violent in recent days as clashes in the region reportedly left nine Malaysian police officers and 19 intruders dead.

"Our security forces were attacked and killed. Malaysians, particularly those in Sabah, are worried about their safety," Malaysian Prime Minister Najib Razak said in a statement carried by the national news agency Bernama announcing the offensive Tuesday.

The standoff has touched on an unresolved territorial question between the Philippines and Malaysia, as well as Manila's efforts to improve relations with Islamic insurgents in the country's south after decades of violence.

The Malaysian forces began their operation around 7 a.m. Tuesday (6 p.m. ET Monday) in Lahad Datu, the district in Sabah where the Filipino men had come ashore, Bernama reported.

A reporter for the news agency in the area said he had heard several explosions from the vicinity of where the Filipinos were believed to be holed up and seen several Malaysian fighter jets flying low nearby.

'The Royal Army'

The Filipino men have called themselves the Royal Army of the Sultanate of Sulu, a former Islamic power in the region that once controlled parts of the Philippines and Borneo. Its influence has since faded, and it is now a clan in the poor, restive southern Philippines.

The sultanate's leadership has been driven by internal power struggles in recent decades, but the squabbling family members appear to have united in the decision to send their followers to Sabah, a move that has alarmed Malaysians and complicated efforts by the Philippines to pursue peace talks with Muslim rebels in its southern islands.