Investigators are examining the possibility the disappearance of a Malaysia Airlines jet with 239 people on board was "an act of piracy", a US official says.
The official, who wasn't authorised to speak publicly, told AP news agency on Friday while other theories are still being looked at key evidence for "human intervention" in the plane's disappearance is that contact with its transponder stopped about a dozen minutes before a messaging system quit.
The official said it was also possible the plane may have landed somewhere.
Another communications system on the plane continued to "ping" a satellite for about four hours after contact was lost with the Boeing 777 during a flight from Kuala Lumpur to Beijing - an indication the plane may have continued to fly on for hours.
Meanwhile a US naval ship and surveillance plane are heading to the Andaman Sea and the Bay of Bengal to search for the airliner.
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A P-8 Poseidon aircraft and a guided missile destroyer, the USS Kidd, were due to aid the international hunt for the jet as the search effort extended further west, Pentagon spokesman Colonel Steven Warren said on Friday.
The Kidd was preparing to search the Andaman Sea and the Bay of Bengal, he said.
"The P-8 will be searching a much larger search area ... the southern portion of the Bay of Bengal and the northern portion of the Indian Ocean".
The Boeing 777 vanished off radar early last Saturday over the South China Sea.
Its fate has vexed investigators and Malaysia authorities have dramatically expanded the scope of the search.
The hunt initially focused on the South China Sea east of Malaysia - along the jet's intended route.
But Malaysia's government is now looking at a vast area, with 13 countries involved.
