Government officials said they asked India to join in the search near the Andaman Sea, suggesting they think the jetliner and the 239 people on board might have reached those waters after crossing into the Strait of Malacca, some 400 kilometers (250 miles) from the flight's last known coordinates.
The mystery over the plane's whereabouts has been confounded by confusing and occasionally conflicting statements by Malaysian officials, which have led to allegations of incompetence or even cover-up, adding to the anguish of relatives of those on board the flight, two thirds of them Chinese.
"There's too much information and confusion right now. It is very hard for us to decide whether a given piece of information is accurate," Chinese Foreign Ministry spokesman Qin Gang told reporters in Beijing. "We will not give it up as long as there's still a shred of hope."
Malaysian Defense Minister Hishammuddin Hussein described the multinational search as unprecedented. Some 43 ships and 39 aircraft from at least eight nations were scouring an area of 92,600 square kilometers (35,800 square miles) to the east and west of Peninsular Malaysia.
"It's not something that is easy. We are looking at so many vessels and aircraft, so many countries to coordinate, and a vast area for us to search," he told a news conference. "But we will never give up. This we owe to the families of those on board."
Malaysia Airlines flight MH370 took off from Kuala Lumpur for Beijing early Saturday and fell off civilian radar screens at 1:30 a.m. about 35,000 feet above the Gulf of Thailand between Malaysia and southern Vietnam. It sent no distress signals or any indication it was experiencing any problems.
"All right, good night," were the last words from the cockpit to air traffic control in Malaysia before the plane entered Vietnamese airspace, Malaysian officials told around 400 relatives of the passengers at a meeting Wednesday in Beijing, according to a participant.
Malaysian authorities said Wednesday that a review of military radar records showed plots of what might have been the plane turning back, crossing over the country and flying to the Strait of Malacca, a busy shipping lane west of the narrow nation.
Air force chief Gen. Rodzali Daud said the radar showed an unidentified object at 2:15 a.m. about 200 miles (320 kilometers) northwest of Penang.
"I am not saying it's flight MH370. We are still corroborating this. It was an unidentifiable plot," he said.
Military and government officials said American experts and the manufacturer of the radar systems were examining that data to confirm whether it showed the Boeing 777. Until then, they said the search would continue on both sides of the country.
The plane's sudden disappearance led to initial speculation of a catastrophic incident that caused its rapid disintegration. The possibility now that it may have continued to fly without communicating with the ground would mean its electrical systems, including transponders which allow it to be identified by commercial radar, were either knocked out or turned off, voluntarily or otherwise.
Authorities have not ruled out any possible cause, including mechanical failure, pilot error, sabotage and terrorism. Both the Boeing 777 and Malaysia Airlines have excellent safety records.
Malaysia said India was joining the search in waters near the Andaman Sea. "As of today, we have not found anything, but we are extending (the search) further," Hishammuddin said.
The plane had about two hours of fuel remaining, so there is a chance it could be far from the area currently being searched.
Confusion over whether the plane had been spotted flying west prompted speculation that different arms of the government might have different opinions about its location, or even that authorities were holding back information.
Earlier in the week, Malaysia's head of civil aviation, Azharuddin Abdul Rahman, was asked why the Strait of Malacca was being searched and replied, "There are things I can tell you, and things I can't," suggesting that the government wasn't being completely transparent with its information.
On Wednesday, Gen. Rodzali released a statement denying remarks attributed to him in a local media report that military radar had confirmed the aircraft had made it to the Strait of Malacca. The Associated Press contacted a high-level military official who confirmed the remarks.
Asked about this, Hishammuddin said his government had been transparent from the start.
"There is only confusion if you want to see confusion," he said. "Basically we have been very consistent on what we've been saying the last few days."
If all those on board are confirmed dead, it would be the deadliest commercial air accident in 10 years.
Choi Tat Sang, a 74-year-old Malaysian man, said his family is still holding out hope that the plane and all on board are safe. His daughter-in-law, Goh Sock Lay, 45, was the chief stewardess on the flight. Her 14-year-old daughter, an only child, has been crying every day since the plane's disappearance.
"We are heartbroken. We are continuing to pray for her safety and for everyone on the flight," he said.
Associated Press writers Jim Gomez in Kuala Lumpur, Isolda Morillo in Beijing, Niniek Karmini in Jakarta, Indonesia, and Rod McGuirk in Canberra, Australia, contributed to this report.