Extensive checks on all Chinese passengers on board the missing Malaysia Airlines plane had found no evidence they were involved in the deliberate diversion of the plane, the Chinese ambassador in Kuala Lumpur said on Tuesday.

More than 150 Chinese nationals were among the 239 people on Beijing-bound flight MH370 when it disappeared early on 8 March.

Investigators believe it was diverted deliberately and say they are investigating the background of the crew, ground staff and passengers. They have not ruled out hijacking, sabotage, terrorism or issues related to personal or psychological problems.

China "conducted meticulous investigation into all the [Chinese] passengers, and did not find any evidence for sabotage activity", Huang said at a press briefing that was reported by the South China Morning Post.

China had begun searching its territory for any sign of the plane after the shifting of the search to a huge arc stretching north and south of the equator, Huang said.

The state news agency Xinhua reported that the country's premier, Li Keqiang, rang the Malaysian prime minister, Najib Razak, on Monday asking him to provide Beijing more detailed data and information in a timely, accurate and comprehensive manner.

He added that despite the difficulty of the search, "as long as there is still a gleam of hope we should continue to do our utmost".

On Monday investigators said they believed the last spoken communication from the plane came from its co-pilot.

But in their Monday briefing, Malaysian officials appeared to backtrack on Sunday's statement that the words "All right, goodnight" came after a communications system was turned off.

Hishammuddin Hussein, transport minister, said the first officer, Fariq Abdul Hamid, was believed to have uttered the last words to Malaysian air traffic controllers at 1.19am – two minutes before the plane's transponder, which communicates with the civil radar system, stopped.

The minister said the last aircraft communications addressing and reporting system (Acars) transmission was at 1.07am, but added: "We do not know when it was switched off after that. It was supposed to transmit 30 minutes from then, but that [subsequent] transmission never came through."

It is unclear why investigators appear so certain that the two communications systems were disabled deliberately, rather than malfunctioning. Attention has focused on the crew – particularly the pilots – because of the difficulty of shutting off the systems and because of the way the plane navigated subsequently.

It is also unclear at what point others on board became aware of the plane's diversion.

The Malaysia Airlines CEO, Ahmad Jauhari Yahya, said investigators were looking at mobile phone records to see whether anyone on board tried to make calls or send texts, but so far there was no evidence of attempted contact.

Investigators are working to narrow the last possible observation of flight MH370 after analysis of satellite data revealed that it was in one of two vast corridors: a northern area stretching from the border of Kazakhstan and Turkmenistan to northern Thailand; and a southern range stretching from Indonesia to the southern Indian ocean. Twenty-six countries are now involved in the search for the plane, which officials believe was diverted not long after it took off from Kuala Lumpur at 12.41am on 8 March, bound for Beijing.

Malaysia has asked countries in the search corridors for radar and satellite information, Hishammuddin said, along with specific assets for search and rescue. It has also asked them to share their search plans.

"It remains a significant diplomatic, technical and logistic challenge," he said, adding that he was grateful for the help Malaysia had received.

Indonesia and Australia are leading the search in the southern corridor.

Officials said they were not aware of Malaysian media reports that the plane could have flown as low as 5,000 ft (1,500 metres) after diverting from its course, allowing it to avoid detection by radar. Malaysia's New Straits Times reported that investigators were considering the possibility that the Boeing-777 dropped to 5,000ft or potentially even lower to avoid detection.

It suggested that the aircraft might not have roused the suspicions of those watching military radars if it followed commercial routes. It also cited unnamed sources as saying the plane had flown low over the Malay peninsula.

It is unclear where the altitude estimate originated; experts said that if it came from radar data, it could well prove incorrect.

Sidney Dekker, of the Safety Science Innovation Lab at Australia's Griffith University, an expert on aviation safety, noted: "Particularly over oceanic areas, radar coverage is extremely unreliable and partial." He noted that flying at 5,000ft would result in a dramatic increase in fuel use, shortening the plane's range.

Jason Middleton, head of the aviation department at New South Wales University, said avoiding radar was a well-known technique.

He added: "Radar goes in a straight line. If you are in the shadow of a mountain or even the curve of the earth – if you are under the radar beam – you can't be seen.

"The further [radar beams] go out the weaker they are and the further they need to come back. Radars have dead zones which are low and also which are far away." However, pilots might also fly low on their way in to a landing, if the cabin was depressurised – to prevent crew and passengers suffering oxygen deprivation – or if they were suffering mechanical problems.

Kazakhstan's civil aviation authority told the BBC it was not possible for the plane to have reached its airspace undetected, noting that it would have had to fly over China, India and other countries. Pakistan's civil aviation authority said checks of its radar recordings found nothing connected to the flight.

Military radar systems can also be limited in their coverage and may not always be in use.

In a statement on Monday, China's foreign ministry, which had more than 150 citizens on the flight, said Malaysia must "immediately" expand and clarify the search.

The English edition of the Global Times – a state-run populist tabloid – ran a scathing commentary on the search effort, accusing Malaysia of incompetence and suggesting it might need to hand over responsibility for the search operation.

Three French investigators have joined the multinational team in Kuala Lumpur. Investigators from the US, UK and China are already involved and Hishammuddin denied a New York Times report that Malaysia had refused to accept large-scale American assistance.

The different teams bring complementary expertise: the French team has knowledge of deep sea rescue due to the 2009 search for Air France flight 447, while the US has advanced data analysis techniques. Officials have said that in some cases – for example, determining the new search area – multiple teams have worked independently so that their calculations can be checked against each other.

Families of the 239 on board have said that investigators' belief that the plane was diverted deliberately has given them fresh hope that the passengers and crew might have survived.