For international airlines that won't turn over advance lists of passengers to be screened for possible terrorists, it's go time.

Customs Commissioner Robert Bonner sent a letter to 58 airlines, telling them that they will face heightened inspections if they have not complied with the new measures by Thursday, even though a new law gives the airlines until next year to start providing the information.

The letter was sent to airlines that, as of Nov. 21, were not providing Customs with advance passenger information. Those airlines include: Saudi Arabian Airlines, Aeroflot Russian Airlines, Air China, Pakistan International Airlines and Ethiopian Airlines.

Many international airlines already offer the information to Customs, but Bonner is urging immediate compliance with the Advance Passenger Information System, which was signed into law last week and gives carriers 60 days to comply.

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• Related: Non-Compliant Carriers

If not, Bonner said that on Thursday the service "will begin heightened inspections of international flight that pose a national security risk because they do not transmit APIS data."

The APIS is used to perform database searches on passengers from foreign locations before they arrive in the U.S.

A spokesman for Royal Jordanian, which also received the letter, said the airline is preparing software to help comply with the new law.

"We are also preparing the required system which will be applicable in the near future which includes the passenger's name, family name, date of birth, passport number, date of issue and expiry," said Ghassan Ali, deputy director general. The airline also has asked all travel agents to have such information available after a flight is booked.

Customs has received information voluntarily from participating airlines since 1988 on international air passengers, including names, birth dates, nationality and travel document numbers. The information is collected at the time of departure and transmitted to Customs while flights are en route to the United States. Information also is transmitted to Customs about crew members as well.

Under the voluntary program with the airlines, Customs currently has access to about 85 percent of international flight passenger information. It has no information on domestic flights, and the new law wouldn't change that. The four hijackings on Sept. 11 involved domestic flights.

The data on international soon-to-arrive air passengers are transmitted to a Customs facility in Virginia.

"We recognize that the vast majority of travelers are not a threat to the United States," Bonner said in the letter.

"However, we believe that in the wake of Sept. 11, international flights pose a serious national security risk to the United States if carriers do not provide comprehensive and accurate APIS data," Bonner said. "Accordingly, for international flights where such data is not provided in advance to the U.S. Customs Service by the carrier, it will be necessary for Customs to address the security risk that such flights pose by, among other things, inspecting all hand-carried and checked baggage on every flight arriving in the United States."

The Associated Press contributed to this report.