There were indications that the Boeing 777-200 passenger jet bound for Beijing turned around shortly before disappearing near the border between Malaysian and Vietnamese airspace, Malaysian armed forces chief Zulkifeli Zin told reporters.
The area for search and rescue operations had been expanded to allow for the possibile change of course, he said.
Chinese family makes contact with missing passenger's phone
The family of a Chinese passenger missing on flight MH370 have made contact with his mobile phone and Malaysian authorities are trying to use the signal to locate the plane, Chinese state television reported on Sunday.
Beijing Television said it also called the man's mobile phone and appeared to connect with it before the call was cut off.
The plane disappeared early Saturday as it was carrying 227 passengers, most of them Chinese, and 12 crew to Beijing from Kuala Lumpur.