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The United States said Tuesday it has revoked "additional visas" since the Christmas Day plane plot amid continuing fears of terror attacks.
"Additional visas have been revoked for people that we believe have suspected ties to terrorism," US State Department spokesman PJ Crowley told reporters.
"It's based on our assessment of ... the risk associated with specific individuals, on information that we have, you know, from various sources," Crowley said.
But he declined to say how many visas have been revoked or from which countries the people concerned were from. Asked if the revocations of visas concerned particular regions, he replied: "It's global."
The Obama administration has tightened security procedures for passengers flying to the United States after a 23-year-old Nigerian, Umar Farouk Abdulmutallab, allegedly tried to bring down a Northwest Airlines jet.
US prosecutors charged that Abdulmutallab tried to blow up Flight 253 using a device containing the explosive PETN, also known as pentaerythritol, but was overpowered by passengers.
Allegedly stitched into his underwear, it was not spotted by the traditional metal detectors. It failed to go off properly, but sparked an on-board fire that was swiftly put out.
President Barack Obama's administration has had to fend off criticism of lax safety and intelligence procedures exposed by the foiled bombing attempt.
