HANOI (AFP) - Mystery surrounded the identity of two passengers who appear to have used stolen European passports to board Malaysia Airlines flight MH370 that vanished carrying 239 people, as the search for the aircraft failed to yield a breakthrough on Sunday.
More than 30 hours after air traffic control lost communication with the Boeing 777, concerns were mounting over the possible security breach, as authorities in South-east Asia said there was still no sign of the plane after the search resumed at sea.
Flight MH370 disappeared about an hour after leaving Kuala Lumpur en route to the Chinese capital. A total of 153 Chinese nationals were on board, and anguished relatives camped out at Beijing airport bemoaned the lack of news on Sunday.
"The airline company didn't contact me, it was a friend," a middle-aged woman surnamed Nan told reporters, holding back tears, after finding out her brother-in-law was on the flight.
"I can't understand the airline company. They should have contacted the families first thing. I don't have any news. I'm very worried," she said.
Vietnamese boats reached the scene of two large oil slicks detected overnight but found no sign of the plane, army deputy chief of staff Vo Van Tuan told AFP early Sunday, adding search aircraft were deployed at first light to scour the area.
The passenger jet slipped off radar screens somewhere between Malaysia's east coast and southern Vietnam early Saturday morning.
However, Azharuddin Abdul Rahman of Malaysia's Department of Civil Aviation said he "could not confirm" the existence of the oil slicks.
"The rescue operation continued last night until this morning...we have not been able to locate or see anything," he told a news conference on Sunday.
Asked about the stolen passports, he said "we are investigating this at the moment" but declined to give further details.
An Austrian named Christian Kozel had his passport stolen in Thailand in 2012, while Italian Luigi Maraldi lost his to theft last year, also in Thailand, officials and reports.
Despite their names being on the passenger manifest, neither man was on the plane to Beijing, officials said.
In Washington, a US administration official said authorities were aware of the passports issue.
"We have not determined a nexus to terrorism yet, although it's still very early and that's by no means definitive. We're still tracking the situation," the official said.
Earlier, when asked whether terrorism could have been a factor, Malaysian Prime Minister Najib Razak said: "We are looking at all possibilities but it is too soon to speculate."
Flight MH370 had relayed no distress signal, indications of rough weather, or other signs of trouble. Both Malaysia's national carrier and the Boeing 777-200 model used on the route are known for their solid safety records.
The flight was carrying 227 passengers and 12 crew. A US company based in Texas said 20 of its employees were among the missing passengers - a dozen from Malaysia and eight from China.
Thirty-eight Malaysians and seven Indonesians were aboard, as well as a range of other nationalities including Australian, Indian, American, Dutch, and French.
Malaysia Airlines urged "all Malaysians and people around the world to pray for flight MH370".
The carrier admitted: "It has been more than 24 hours since we last heard from MH370 at 1:30 am. The search and rescue team is yet to determine the whereabouts of the Boeing 777-200 aircraft."
The plane's disappearance triggered a search effort involving vessels from several nations with rival maritime claims in the tense South China Sea.
China, Vietnam, Malaysia, the Philippines and Singapore threw vessels and aircraft into the effort, as did the US Navy with a surveillance plane and a destroyer carrying two helicopters.
"We are setting these (territorial) issues aside in view of humanitarian reasons," Philippine military spokeswoman Lieutenant Cheryl Tindog said.
If the worst is confirmed, it would be the second fatal crash in the nearly 20-year history of the popular Boeing plane. A 777-200 operated by South Korea's Asiana Airlines skidded off the runway after hitting a sea wall in San Francisco last year, killing three people.
Malaysia Airlines has suffered few safety incidents in the past. Its worst occurred in 1977, when 93 people perished in a hijacking and subsequent crash in southern Malaysia.