Police said the video, believed to feature operatives from the militant group Katibah Nusantara speaking under the IS logo, was significant because, if confirmed, it was the first from Islamic State in Malay.
“This threat is very real and my government takes it very seriously,” Najib told a conference on extremism. “This is a challenge that faces us all around the world. We are far from immune to this danger in Malaysia.”
Police said on Sunday they had arrested seven members of an IS cell who were planning attacks across the country. The suspected militants were carrying bullets, books on jihad, IS flags and propaganda videos.
Ten days ago, Malaysia arrested a suspected militant believed to have been planning a suicide attack in Kuala Lumpur.
The video that surfaced online warned Malaysia against the crackdown on IS supporters.
Ayob Khan Mydin Pitchay, director of the police counter-terrorism unit, said the video suggested militants in the country were becoming more organised.
Reuters could not independently verify the video.
“If you catch us, we will only increase in number but if you let us be, we will be closer to our goal of bringing back the rule of the Khalifah (caliph),” said a message on the video, according to Malaysian newspaper the Star.
Security experts in the region believe Islamic State’s footprint is still light in Southeast Asia because militants are jostling to be its regional leader.