Prime Minister Najib Razak says he's "deeply concerned" by the first discovery of mass graves of suspected illegal migrants in northern Malaysia and has vowed to find those responsible.

"I am deeply concerned with graves found on Malaysian soil, purportedly connected to people-smuggling," he said on his Facebook and Twitter accounts.

"We will find those responsible."

Malaysia's home minister announced on Sunday that mass graves were found near jungle camps along the border with Thailand, further evidence of the lethal nature of a human-trafficking trade that has blown up in to a regional crisis.

Police in neighbouring Thailand in early May had found secret human-trafficking camps on their side of the border and dozens of shallow graves. These are the first found in Malaysia.

Officials have yet to say how many bodies are in the Malaysian graves or their precise locations, but the country's police chief was expected to hold a press conference at 11am (1500 AEST) on Monday.

There may be dozens or even hundreds of corpses in the graves, Malaysian media reports have said, citing unidentified sources.

Malaysia's Perlis state, where the graves were found, is near where Thailand found its graves.

The area is said by anti-trafficking groups to be a key stop on a route that funnels migrants from Bangladesh and Myanmar into Malaysia and beyond.

Thailand launched a crackdown on human-smuggling following the discovery of its mass graves.

The move appears to have caused nervous traffickers to abandon their human cargo at sea, leaving boats filled with hundreds of starving migrants seeking to land in Malaysia, Thailand and Indonesia.

After initially turning them away, Malaysia and Indonesia last week bowed to international pressure, saying they would admit boat people pending their repatriation or resettlement elsewhere.