The international hunt for the missing Malaysia Airlines flight MH370 expanded to cover 27,000 square nautical miles on Wednesday as a third potential last sighting added to the confusion over its movements.

India became the 12th country to say it would join the search, indicating how far north the operation had been extended.

The plane was heading north-east to Beijing when it took off from Malaysia in the early hours of Saturday morning. Officials have said it may have turned around and headed back to Kuala Lumpur when it was lost.

On Wednesday, in a new twist to the mystery, officials suggested the plane may have been detected on military radar at 2.15am on Saturday, 200 miles north-west of Penang – a point which is not only west of the Malay peninsula, but so far north that it would be beyond the coast of Thailand. It was the third possible final time and location officials have given.

"We are not saying this is MH370. It's an unidentified plot," said air force chief Rodzali Daud at a press conference in Kuala Lumpur.

Malaysian authorities are facing growing criticism about muddled and sometimes contradictory briefings.

China's foreign ministry spokesman Qin Gang told reporters: "Right now there is a lot of information, and it's pretty chaotic, so up to this point we too have had difficulty confirming whether [detection over the strait of Malacca] is accurate or not."

Vietnamese officials had said they were calling off their air search and scaling back their sea search pending further information from Malaysia, but have since announced they will resume a full-scale search.

Malaysia's transport minister, Hishammuddin Hussein, told reporters: "With each day that passes, I fear search and rescue becomes just search – but we will never give up hope."

He insisted that authorities had been consistent and transparent, adding: "It is only confusion if you want it to be seen to be confusion."

He told reporters: "We have nothing to hide."

Five days after Beijing-bound MH370 disappeared not long after taking off from Kuala Lumpur at 12.41am on Saturday, questions regarding its final moments have if anything multiplied.

The last certain contact with the plane was at around 1.30am, when it was flying over the South China Sea, between Malaysia and Vietnam. Subsequent readings come from military radars which can detect civilian planes but do not communicate with them, meaning that they cannot identify a particular flight. It is not clear if these radar readings were cross-checked with other information.

On Tuesday it was reported that the Malaysian air force chief said the plane had been detected at 2.40am close to Pulau Perak, an island in the strait of Malacca – meaning it had not only turned back but flown right across the peninsula. Rodzali Daud subsequently denied making that comment, but it is not clear if the authorities are discounting the possible presence of MH370 over the strait or simply cannot confirm it.

Experts stress it is far too early to be certain what happened. If both the military radar spots are correct, then they could be consistent with the plane turning back and attempting to navigate back to Kuala Lumpur along the west coast of the peninsula.

Earlier on Wednesday, pressed by relatives of Chinese passengers on what information the military had given civil officials, the Malaysian government's envoy to China told them now was "not the time" to reveal it, Singapore's Straits Times reported.

He did disclose that the last words heard from the flight were: "All right, good night" – the crew's response to Malaysian air traffic controllers telling them the flight was entering Vietnamese airspace and that air traffic controllers from Ho Chi Minh City would take over.

Two-thirds of those on board the Boeing-777 were Chinese, and China has repeatedly urged Malaysia to speed up search efforts as well as sending more ships and aircraft to assist. It is under pressure as public concern grows about the state of the operation, and relatives have already vented their frustration at Chinese officials.

The Washington Post cited one popular post, which has been shared thousands of times by Chinese microbloggers: "Vietnam keeps discovering. Malaysia keeps denying. China keeps sending things on the way. Journalists keep waiting at the Lido hotel [where relatives are waiting]. Family members keep being in pain … But where is the plane?"

Malaysia Airlines has confirmed it received a warning from the Federal Aviation Administration in November, urging airlines to look out for corrosion under the skin of the fuselage of Boeing 777s.

Chief executive Ahmad Jauhari Yahya added: "We ensure that all our aircraft are airworthy and comply with all the [circulars] issued by the manufacturers."

Experts have said the aircraft has a strong safety record.