China has urged Malaysia to "step up its efforts" in the search for the Malaysia Airlines passenger plane that disappeared on Saturday.
Malaysia said it was widening the hunt, after days of searching found no trace of the plane or the 239 people on board - most of whom were Chinese.
Rescue teams from nine countries will now scour areas stretching from the Malacca Strait to the South China Sea.
Beijing-bound flight MH370 vanished shortly after it left Kuala Lumpur.
Relatives of the missing passengers have been told to prepare for the worst.
Chinese foreign ministry spokesman Qin Gang had earlier urged the Malaysian authorities to sharpen its search efforts.
"We have a responsibility to demand and urge the Malaysian side to step up search efforts, start an investigation as soon as possible and provide relevant information to China correctly and in a timely manner," he said.
Patience appears to be wearing thin in the search for the missing aeroplane, says the BBC's Celia Hatton in Beijing.
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.
"If we go to Malaysia, we can do nothing but wait, just like 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.
Earlier, the Malaysian authorities said they had 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 not Malaysian.
International police agency Interpol has confirmed the passengers were travelling with Italian and Austrian passports stolen in Thailand years ago.
At a news conference on Monday, Malaysia's civil aviation chief Azharuddin Abdul Rahman said the two men were "not Asian-looking men".
He insisted that all security protocols had been complied with before the plane took off.
Experts say the presence of two passengers with stolen passports is a breach of security, but is relatively common in the region and could relate to illegal migration.
Some 40 ships and 34 aircraft from nine different nations are taking part in the search in the seas off Vietnam and Malaysia.
Commander William Marks from the US Seventh Fleet, which is taking part in the search, said he expected the plane's flight recorders to be floating in the water.
He said the recorders, also known as "black boxes", were fitted with radio beacons that can be picked up by radar.
Despite a wide search, radar had not so far picked up any signals, he said.
None of the debris and oil slicks spotted in the water so far have proven to be linked to the disappearance.
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 terrorist attack. Counter-terrorism agencies and the FBI are involved in the operation.
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.
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.