Malaysia's acting transport minister, Hishamuddin Hussein: Search and rescue 'the main focus'. Photo: BBC NEWS

Nearly three days after the jet went missing, an international effort has still not found the plane's wreckage.

None of the debris and oil slicks spotted in the water so far have proven to be linked to the disappearance.

Flight MH370 vanished from radar almost three days ago en route to Beijing from Kuala Lumpur, with 239 people on board.

Relatives of the missing passengers have been told to prepare for the worst.

China said the Malaysian authorities needed to "step up their efforts" to find the missing airliner, which had more than 150 Chinese nationals on board.

"We... have a responsibility to demand and urge the Malaysian side to step up [its] search efforts, start an investigation as soon as possible and provide relevant information to China correctly and in a timely manner," said Chinese foreign ministry spokesman Qin Gang.

Patience appears to be wearing thin in the search for the missing airplane, says the BBC's Celia Hatton in Beijing.

Earlier, the Global Times, a prominent Communist Party newspaper, issued a sharply worded editorial arguing there were "loopholes" in the work of Malaysian airlines and security authorities, noting that "until yesterday [Sunday], the Malaysian government could not even ensure accurate information about the [plane's] passengers".

The Malaysian authorities are attempting to address Chinese concerns - they have reissued a pledge to fly worried family members to Kuala Lumpur so they can be closer to the search efforts, our correspondent adds.

But one victim's relative - Guo Qishun, whose son-in-law was on the plane - said he did not see the point of flying to Malaysia.

"We don't want to go to Malaysia now. There is no result from the search operation yet. If we go to Malaysia, we can do nothing but wait, just like what we are doing in Beijing now. If we go to Malaysia, who can we rely on? Most of us don't speak English," he told the Associated Press news agency.

Malaysian authorities have identified one of the two men travelling on the missing plane on stolen passports.

Police chief Khalid Abu Bakar said they could not reveal his identity, but confirmed the man was a non-Malaysian.

Malaysia's acting transport minister, Hishamuddin Hussein, said all of the relevant information concerning those two passengers had now been passed on to the various national intelligence agencies which were investigating the matter.

International police agency Interpol has confirmed the passengers were travelling with Italian and Austrian passports stolen in Thailand years ago.

The presence of two passengers with stolen passports is a breach of security, but could relate to illegal migration, observers say.

Search continues

The search area has been widened to include waters in the Strait of Malacca.

Commander William Marks from the US Seventh Fleet, which is taking part in the search, says he expects the plane's flight recorders to be floating in the water.

"In calm seas, if there were a soccer ball [football] or a basketball floating in the water, the radar could pick it up. They [flight recorders] typically have a radio beacon and so for example our P3 [radar] - if they are flying within a certain range of that - will pick up that radio beacon. We have not yet picked up anything, but that's typically what those black boxes contain."

There are now 40 ships and 34 aircraft from nine different nations taking part in the search in the seas off Vietnam and Malaysia.

US Navy officials earlier said their aircraft had not seen any debris associated with commercial aircraft wreckage.

Flight MH370 left Kuala Lumpur for Beijing at 00:41 local time on Saturday (16:41 GMT on Friday). But radio contact was lost at 17:30 GMT, somewhere between Malaysia and Vietnam.

Officials say they still have no idea what went wrong.

Malaysian military officials said on Sunday they were widening the search area because of indications the plane, a Boeing 777-200ER, may have turned back from its scheduled route shortly before vanishing from radar screens.

Investigators are looking at all angles, including a possible terror attack. Counter-terrorism agencies and the FBI are involved in the operation.

Five passengers booked on the flight did not board, and their luggage was consequently removed.

The passengers on the flight were of 14 different nationalities. Two-thirds were from China, while others were from elsewhere in Asia, North America and Europe.

British engineering company Rolls-Royce has confirmed that the Malaysia Airlines Boeing 777 was powered by two Trent 800 engines, built in Derby.

Malaysia Airlines is the country's national carrier, flying nearly 37,000 passengers daily to some 80 destinations worldwide.

On Monday, shares in Malaysia Airlines fell 18% to a record low.