WASHINGTON (AFP) - The United States on Wednesday urged Malaysian authorities to carry out a full investigation into the grisly discovery of mass graves at human-trafficking camps and charge those behind the killings.

"We understand Malaysian law enforcement and security authorities are actively investigating these findings, and we encourage a transparent, credible and expeditious effort," a State Department spokesman told AFP.

"We urge Malaysian authorities to investigate fully these deaths and camps, and prosecute those responsible."

A top Malaysian official was quoted as saying Wednesday that Malaysian police were investigating 12 of their officers to determine whether they had links to the graves found in the remote north.

Malaysian police said earlier this week that a total of 139 gravesites and 28 recently abandoned camps had been found along the northern border with Thailand, capable of housing hundreds of people.

Police believe they were used by human traffickers and smugglers. It is still unclear how many bodies the graves hold.

Washington is working with "the affected governments and international organizations in the region to combat these crimes and protect the victims," the State Department official said.