(Corrects typo in first paragraph)
BEIJING, March 10 (Reuters) - China urged Malaysia to step up the search for a Malaysia Airlines jetliner that went missing with 239 people on board, about two-thirds of them Chinese, and said it has sent security agents to help with an investigation into the misuse of passports.
Questions have mounted over possible security lapses and whether a bomb or attempted hijacking could have brought down the Beijing-bound plane, after Interpol confirmed at least two passengers used stolen passports and said it was checking whether others aboard used false identity documents.
As dozens of ships and aircraft from seven countries scoured the seas around Malaysia and south of Vietnam, Chinese Foreign Ministry spokesman Qin Gang said it was too early to know what may have caused the plane to vanish but that family of the passengers deserved an explanation as soon as possible.
"This incident happened more than two days ago, and we hope that the Malaysians can fully understand the urgency of China, especially of the family members, and can step up the speed of the investigation and increase efforts on search and rescue," Qin told reporters at a daily news briefing.
China's Public Security Ministry had sent a team to Malaysia to look into the use of the two stolen passports.
"We cannot confirm at present who it was who misused these two passports," he said, adding the incident "should certainly attract our greatest vigilance".
As they were Italian and Austrian passports, they would not have needed a visa for a stay in Beijing that did not exceed 72 hours, Qin said.
He would not speculate on whether foul play was behind the aircraft's disappearance.
Flight MH370 vanished from radar screens in the early hours of Saturday, about an hour into its flight from Kuala Lumpur, after climbing to a cruising altitude of 35,000 ft (10,670 metres).
No distress signal was sent, which experts said suggested a sudden catastrophic failure or explosion. But Malaysia's air force chief said radar tracking showed it may have turned back from its scheduled route before it disappeared.
About two-thirds of the 227 passengers and 12 crew now presumed to have died aboard the plane were Chinese. The airline said other nationalities included 38 Malaysians, seven Indonesians, six Australians, five Indians, four French and three Americans.
The passenger manifest issued by the airline included the names of two Europeans - Austrian Christian Kozel and Italian Luigi Maraldi - who were not on the plane. Their passports had been stolen in Thailand during the past two years.
A friend of one passenger, speaking to reporters after a meeting with China's civil aviation authority and government officials in Beijing, said passengers' families were growing impatient.
"The family members are really not happy. They feel like they have waited far too long," the man, surnamed Zhou, said.
"The main thing they are interested in is whether there is anyone left alive or not." (Reporting by Ben Blanchard, Natalie Thomas and Megha Rajagopalan, Writing by Michael Martina; Editing by Nick Macfie)