KUALA LUMPUR, Malaysia—Malaysia's prime minister said he believes that "deliberate action" caused the disappearance of Malaysia Airlines Flight 370, and authorities are now focusing on two areas in the search for the plane.

In the first definitive comments from the nation's government on how the jet came to vanish March 8, Najib Razak said at a news conference Saturday that data relayed by the plane to a satellite has confirmed it turned from its original course. He added that the last satellite communication from...