Malaysian Prime Minister Najib Razak has pledged to fight extremism and terrorism in Southeast Asia, where officials said Islamic State militants have been working to co-opt local terror groups.

Speaking at a business conference in Jeddah, Saudi Arabia, Najib said late Tuesday that the two countries will remain "inseparable partners" against terror groups that have caused havoc in the Middle East and around the world.

"We stand shoulder to shoulder with you in this fight," Najib said, according to the Star Online newspaper. "And we will never tire in the battle to defend the true, authentic Islam."

Besides the predominantly Muslim Malaysia, Southeast Asia is home to Indonesia, which has the world's biggest Muslim population. They are grappling with deadly extremism with militants from both countries known to have fought in the Middle East. Neighboring Philippines is also fighting a band of militants that have pledged to fight under the Islamic State group, also known as ISIS. 

In January, six Indonesians, including four attackers were killed in blasts in Jakarta. This came after the arrest of nine militants allegedly plotting to carry out bombings in Indonesia. Last year, a Malaysian terrorist named Zulkifli bin Hir, also called Marwan, was killed by Philippine troops in a clash in the southern region of Mindanao. He had a $5 million bounty on his head offered by U.S. authorities.

Najib stressed that bilateral and defense ties with Saudi Arabia remained strong, with the Malaysian armed forces having joined Northern Thunder, the biggest military exercise in the Middle East.

Najib said the two Muslim countries were "like brothers" as he thanked the Saudi royal family for extending its help, and for hosting the Forum, which gathered about 20 countries seeking investment opportunities offered by the kingdom. Najib is fighting corruption allegations stemming from the $681 million discovered in his bank accounts. He had said the money came from Saudi Arabia's royal family.

Southeast Asian militants, including the Abu Sayyaf in the jungles of the Philippines, have pledged allegiance to ISIS and vowed to carry out deadly attacks to further its cause.

Translated as the "bearer of the sword," the Abu Sayyaf has carried out some of the region's deadliest attacks, including the 2003 fire bombing of a ferry that left more than 100 dead on Manila Bay, and countless bombings and attacks in the Philippines.

It has also carried out raids into Malaysia, grabbing Asian and western hostages they later ransomed off for millions of dollars. The Philippine government has been working with its neighbors, including Malaysia, to defeat the group, which is known to operate in the country's vast, porous southern regions.