Malaysia's handling of Flight MH370's disappearance faced new criticism on Monday after an interim investigation uncovered a dud beacon battery and more potential missed opportunities to track the plane a year ago.

The international investigation team set up by Malaysia released its interim report on Sunday, the first anniversary of the disappearance. It contained no new clues on what caused the plane to vanish with 239 passengers and crew aboard.

But analysts and next of kin raised questions over some findings, including the revelation that the beacon battery on the flight data recorder had expired more than a year before the flight.

"I hope the international aviation body punishes Malaysia Airlines for non-compliance with regards to the expired battery," said Lim Wee Hoon, a Malaysian national whose brother-in-law was aboard.

She called the report part of a continuing Malaysian "cover-up".

The battery discovery "raises issues over the integrity of maintenance" at the carrier, said Greg Waldron, Asia managing editor for Flightglobal magazine.

"Did that play a role in the inability to find the plane? We really don't know if that was a factor," he said.

The plane is believed to have crashed in the remote southern Indian Ocean, but nothing has been found despite an expensive and difficult search effort led by Australia.

- 'Maintenance oversight' -

In a statement on Monday, Malaysia Airlines called the battery issue a maintenance "oversight".

But the airline said the battery on the separate cockpit voice recorder -- good for 30 days once activated -- was up-to-date and would have transmitted a signal once it hit water.

It added that the carrier had "taken significant steps to improve safety" since MH370 was lost.

The plane vanished at night over the South China Sea after turning away from its north-bound route from Kuala Lumpur to Beijing.

The government and state-controlled airline came under fire in the crisis's chaotic early days for a series of confusing and contradictory statements.

Previously, Malaysian authorities said the plane dropped off civilian radar -- after systems allowing its position to be tracked appear to have been disabled on board -- but remained on military radar for some time as it flew west toward the Indian Ocean.

Malaysia's military did not act, infuriating relatives over the missed opportunity to follow the plane.

But the investigative report, citing radar records, said a blip "consistent" with the military radar data actually did appear on civilian screens several times for about 30 minutes after the plane diverted and doubled back over Malaysia.

The report did not mention any reaction by civilian authorities to this blip.

Waldron said it "looks like it was MH370," but the report does not clearly state that it was.

- Dozing supervisor -

The report also included a transcript in which a Kuala Lumpur air-traffic controller, pressed by Malaysia Airlines for information on the plane four hours after it vanished, said he would go and "wake up my supervisor" to help.

Responding to the report, Voice370, an international association of relatives, lambasted authorities for a response "full of chaotic confusion", saying that "tremendous precious rescue time was lost" to intercept the plane.

"(We) believe this has contributed much to the fact now that we know nothing about the whereabouts of our loved ones," it said in an emailed statement to AFP.

The director of Malaysia's Civil Aviation Department, Azharuddin Abdul Rahman, declined comment to AFP on the radar issue.

Transport Minister Liow Tiong Lai said in a statement Monday the contents of the report were a matter of "serious concern".

"We will take stern action once we have analysed the factual information, particularly from the perspective of civil aviation-related protocols and processes," it added.

Theories on the cause of the disappearance have included a pilot going rogue, mechanical problems, or hijacking.

The investigators, focused on air-safety issues, did not look into a possible hijack.

But the report said they looked thoroughly into the backgrounds of the 12 crew, finding nothing suspicious, and no red flags were seen in checks of the maintenance histories of the plane's key mechanical systems.

The report stressed that the investigation remained a work in progress until wreckage and data recorders can be found.

The search of the suspected Indian Ocean crash zone is expected to end in May.

But while both Malaysia and Australia say they are committed to finding the plane, neither has given a clear indication what will happen after May.