She said the pair booked the tickets through China Southern Airlines, but she had no information on where they bought them.
As holders of EU passports with onward flights to Europe, the passengers would not have needed visas for China.
Interpol said it and national investigators were working to determine the true identities of those who used the stolen passports to board the Malaysia Airlines flight.
White House Deputy National Security Adviser Tony Blinken said the U.S. was looking into the stolen passports, but that investigators had reached no conclusions.
In addition to the plane's sudden disappearance, which experts said was consistent with a possible onboard explosion, the stolen passports strengthened concerns about terrorism as a possible cause. Al-Qaida militants have used similar tactics to try to disguise their identities.
Still, other possible causes included a catastrophic failure of the plane's engines, extreme turbulence, or pilot error or even suicide. Establishing what happened with any certainty will need data from flight recorders and a detailed examination of any debris, something that will take months if not years.
Malaysia's air force chief, Rodzali Daud, said radar indicated that before it disappeared, the plane may have turned back, but there were no further details on which direction it went or how far it veered off course.
"We are trying to make sense of this," Daud said at a news conference. "The military radar indicated that the aircraft may have made a turn back, and in some parts this was corroborated by civilian radar."
Malaysia Airlines Chief Executive Ahmad Jauhari Yahya said pilots are supposed to inform the airline and traffic control authorities if the plane does a U-turn. "From what we have, there was no such distress signal or distress call per se, so we are equally puzzled," he said.
A total of 34 aircraft and 40 ships from Vietnam, Malaysia, Thailand, Australia, Singapore, Indonesia, China and the United States were deployed to the area where ground controllers lost contact with the plane, the maritime border between Malaysia and Vietnam.
Of the 227 passengers and 12 crew members on board, two-thirds were Chinese, while the rest were from elsewhere in Asia, Europe and North America, including three Americans.
Family members of Philip Wood, a 50-year-old IBM executive who was on board the plane, said they saw him a week ago when he visited them in Texas after relocating to Kuala Lumpur from Beijing, where he had worked for two years.
"There is a shock, a very surreal moment in your life," said Wood's brother, James Wood.
The other two Americans were identified on the passenger manifest as 4-year-old Nicole Meng and 2-year-old Yan Zhang.
After more than 30 hours without contact with the aircraft, Malaysia Airlines told family members they should "prepare themselves for the worst," Hugh Dunleavy, the commercial director for the airline, told reporters.
Finding traces of an aircraft that disappears over sea can take days or longer, even with a sustained search effort. Depending on the circumstances of the crash, wreckage can be scattered over many square kilometers (miles). If the plane enters the water before breaking up, there can be relatively little debris.
A team of American experts was en route to Asia to be ready to assist in the investigation into the crash. The team includes accident investigators from the National Transportation Safety Board, as well as technical experts from the Federal Aviation Administration and Boeing, the safety board said in a statement.
Malaysia Airlines has a good safety record, as does the 777, which had not had a fatal crash in its 19-year history until an Asiana Airlines plane crashed last July in San Francisco, killing three passengers, all Chinese teenagers.
Brummitt reported from Hanoi, Vietnam. Associated Press writers Rod McGuirk in Canberra, Australia; Didi Tang, Gillian Wong and Louise Watt in Beijing; Joan Lowy in Washington; and Scott Mayerowitz in New York contributed this report.
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