Malaysian authorities have reiterated they believe there was a “deliberate action” by someone on board the missing Malaysian airliner to divert it from its scheduled flight path.

“MH370’s movements were consistent with deliberate action by someone on the plane,” Malaysia’s acting transport minister Hishammuddin Hussein said on Tuesday night.

The comments refute growing speculation that a mechanical failure led to cabin depressurisation that caused the pilots to become unconscious after turning the plane around over the South China Sea.

Mr Hishammuddin said international investigators and Malaysian authorities believe the plane was subjected to deliberate action until the time it left military radar after it had turned back from its route to Beijing and travelled back over peninsula Malaysia to the Straits of Malacca.

Meanwhile, the final three seconds of contact between the cockpit of MH370 and Kuala Lumpur air traffic control provides no clue as to what happened to the plane.

Malaysian authorities released a transcript of the conversations on Tuesday amid criticism they have been withholding information about the plane’s disappearance, which has fuelled anger among the relatives of the 239 people who were on board.

At 1.19 and 24 seconds air traffic control told the pilots “Malaysian three seven zero contact Ho Chi Minh 120 decimal 9 good night.”

This was a sign-off from Kuala Lumpur as the plane entered the area of responsibility for Vietnam air space.

Someone in the cockpit replied “good night Malaysian three seven zero” at 1.19 and 29 seconds. That was the last contact with the plane.

Malaysian officials said initially they believed the final words were spoken by the plane’s first officer Fariq Abdul Hamid, 27, who was making his first unsupervised flight in a Boeing 777.

But Mr Hishammuddin said “police are working to confirm this belief and forensic examination of the actual recording is on-going.”

From 12.36 am when MH370 signed on “Tower MAS370 morning” until the final communication at 1.19 and 29 seconds there appears to be nothing abnormal in the conversations.

The words used from the plane and air traffic control were polite language normally used by pilots.

As the plane taxied on the tarmac of Kuala Lumpur airport conversations were “garbled” on four separate occasions, according to the transcript.

Air traffic control approved take-off at 12.40am with the call “370 32 right cleared for take-off. Good night.”

MAS370 replied: “32 right cleared for take-off MAS370. Thank-you goodbye.”

The MH370 cockpit reported the plane’s altitude six times between 12.42 and 12.50am when someone says” “flight level three five zero Malaysian three seven zero.”

MH370 reported its altitude at 1.01am and then repeated it six minutes later. Air traffic control responded each time.

Before releasing the transcript to the media it was shown to the relatives of those who were on board in Beijing and Kuala Lumpur.

Mr Hishammuddin said the transcript was held for more than three weeks as part of the police investigation into the plane’s disappearance.

Desmond Ross, an Australian commercial pilot and security aviation expert, said the conversations appear consistent with the flight.

He said it is interesting there were “garbled” communications before take-off.

“I wonder if they were having some sort of radio difficulties from the outset which may have become more of a problem later in the flight,” he said.