The U.S. government strongly believes that North Korean agents murdered the estranged half-brother of North Korean leader Kim Jong Un in Malaysia, a U.S. government source told Reuters on Tuesday.

U.S. authorities have not yet determined exactly how Kim Jong Nam was killed, according to the source, who did not provide firm evidence to support the government's conclusion.

A South Korean government source said Kim Jong Nam was killed in Malaysia and a Malaysian police official said the cause of his death was not yet known.