Gunmen ambushed and killed five Malaysian policemen as fears mounted that armed intruders from the southern Philippines had slipped into at least three coastal districts on Borneo island, officials said on Sunday.

Two of the attackers were also fatally shot on Saturday night, escalating tensions in Sabah State, where Malaysia’s biggest security crisis in recent years began after about 200 members of a Philippine Muslim royal clan occupied a village last month to claim the territory as their own.

Security forces clashed with the clan members in the coastal area of Lahad Datu on Friday, leaving 12 Filipinos and two Malaysian police commandos dead.

The remaining clan members have refused to budge, while concerns have grown that other groups from the Philippines’ restive southern provinces might enter Sabah, which shares a long and porous sea border with the Philippines that’s difficult to patrol.

A police team was attacked late Saturday while inspecting a settlement in Semporna town, more than 150 km from Lahad Datu, said Malaysia’s national police chief Ismail Omar. Authorities were searching the area for more of the assailants.

Police are also investigating sightings of armed foreigners in military-style clothing in a third Sabah seaside district nearby, Mr. Ismail said.

It was not clear whether the groups in the three areas had links to each other.

Malaysian Prime Minister Najib Razak said on Sunday that army reinforcements have been sent to Sabah, adding that he was confident about their ability to control the situation.

The Filipinos who landed in Lahad Datu on February 9 say ownership documents from the late 1800s prove the territory is theirs. They have rejected repeated calls from both the Malaysian and Philippine governments to leave Sabah, a short boat ride from southern Philippines.

Police dropped leaflets by helicopter over the occupied village on Saturday telling the Filipinos to give up, while the navy bolstered patrols in waters between Malaysia and the Philippines.

Three of the intruders tried to escape late Saturday and were caught, Mr. Ismail said, without elaborating.

The standoff has raised territorial issues in Sabah and the southern Philippines to an immediate national security concern for both countries.

Malaysia’s government, which faces a national election within less than four months, is under pressure to stop cross-border incursions that have resulted in occasional kidnappings by Filipino gunmen in past years.

It could affect how authorities deal with tens of thousands of Filipino migrants living in Sabah, including many undocumented workers, if they become perceived as threats to public safety. Any plan to deport them on a large scale, as Malaysia has sometimes attempted, would be a delicate diplomatic issue.

The crisis could also complicate peace talks brokered by Malaysia between Manila and the Moro Islamic Liberation Front, the main Muslim rebel group in the southern Philippines.

The Lahad Datu group is led by a brother of Sultan Jamalul Kiram III of the southern Philippine province of Sulu.

In Manila, the sultan told reporters that he was worried the violence in Sabah might spread because many Filipinos, especially followers of his sultanate in the southern Philippine, are upset by the killing of their compatriots in Lahad Datu.

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