Malaysia's civil aviation chief said: "This is still a search-and-rescue operation" — rather than a disaster-recovery operation, suggesting hopes of recovering passengers alive.
The next press briefing on the missing aircraft is scheduled for 10 a.m. in Kuala Lumpur.
Flight MH370 disappeared early on Saturday, about an hour into its flight from Kuala Lumpur to Beijing, after climbing to a cruising altitude of 35,000 feet.
About 40 ships and roughly 34 aircraft from countries including the U.S., China, Australia and Singapore are taking part in the search effort for the plane carrying 227 passengers and 12 crew members.
Malaysia airlines has said people of 14 nationalities were among the passengers, including at least 152 Chinese, 38 Malaysians, seven Indonesians, six Australians, five Indians, four French and three Americans.
International police agency Interpol confirmed on Sunday that two passengers had boarded the flight using stolen Austrian and Italian passports, raising fears about a possible terrorist attack.
Separately, Malaysian authorities said on Monday that five passengers who had bought tickets failed to board the flight.
Authorities also warned that a stolen-passport syndicate could have been involved. Their identities have not yet been confirmed, but the authorities said they weren't "Asian-looking".
"The two passengers traveling on false passports is a red flag…That's undoubtedly where the focus of the investigation is going to turn as soon as we know more about the location of the crash site," said Richard Aboulafia, vice president of analysis at Teal Group Corporation.
"I think fundamentally, they are searching in the wrong location," Scott Hamilton, founder of aviation consultancy Leeham, told CNBC.
"If the plane has gone down anywhere along the intended flight path, I feel very confident we would have seen debris. The fact that you haven't seen debris suggests to me the airplane crashed some place else, whether on land or some place else in the water," he said.
At the Monday press conference, the authorities said that debris had been noticed by an aircraft south of Hong Kong. Ships are being dispatched to investigate, but the results will not be known until Tuesday.
Fuad Sharuji, vice president of operations control at Malaysia Airlines, said the airline had received several pieces of information that might be related to the missing aircraft, but had so far proved inconclusive.
Vietnam fueled speculation for a breakthrough when it announced it had sent helicopters to investigate a floating yellow object, thought to be a life raft. But the country's civil aviation authority later said the object had proved to be a "moss-covered cap of cable reel."
Reuters meanwhile reported a senior source as saying that officials investigating the disappearance suspected the jet might have disintegrated in mid-flight.
(Read more: Five things to know about the jet)
No distress signal was sent from the lost plane, which experts said suggested a sudden failure or explosion. The jet last had contact with air traffic controllers 120 nautical miles off the east coast of the Malaysian town of Kota Bharu, about an hour after take-off.
Malaysia's air force said that radar-tracking showed the plane might have turned back from its route before it went missing.
"Certainly something unexpected happened and it had to be catastrophic in nature in order to prevent the crew from communicating it," said John Goglia, former member of the National Transportation Safety Board in the U.S.
"That doesn't rule out mechanical failure and it doesn't rule out some sort of activity by person or persons unknown."