LONDON -- Shipping company UPS has been barred from moving air cargo through some U.K. facilities because of security flaws, the British government said Friday after it carried out security tests.

The Department for Transport gave no details on the security issues and didn't identify the locations involved. Atlanta-based UPS said the restrictions were the result of a government security check rather than a specific threat.

"Following careful consideration, the department has restricted the number of sites in the U.K. at which UPS Ltd. are permitted to screen air cargo until it has satisfied current security requirements," the U.K. transport department said in a statement.

The department said it could not give any details about the blocked sites for security reasons.

UPS spokeswoman Susan Rosenberg said six of the company's British facilities -- including its main air hub at East Midlands airport in central England -- were still operating. She would not say how many facilities had been shut down.

UPS told customers Friday that shipments from Britain were being delayed. It said "areas of concern" were found during a government review of "UPS procedures and employment documentation related to security."

It said some facilities were temporarily taken offline, which has led to some delays in the movement of packages.

"UPS has activated contingency plans, communicated with customers and expects service levels to return to normal early next week," the company said in a statement. "UPS continues to work with all agencies around the world to maintain and enhance security and to balance necessary protections with the free flow of commerce, just as we have always done."

No other air freight companies were mentioned in the U.K. government statement.

The vulnerability of air cargo to terrorist attacks is a major worry for international security agencies.

Last October, two bombs were sent disguised in toner cartridges on cargo flights from Yemen bound for the United States. One was discovered at a FedEx cargo facility in Dubai, the other at the UPS depot at East Midlands Airport, 100 miles north of London.

Officials said the bombs were viable and could have exploded in mid-flight. Al Qaeda in the Arabian Peninsula was blamed for the plot, which was foiled by a tip-off from Saudi intelligence.

Britain suspended all unaccompanied air cargo from Yemen and Somalia after the thwarted plot, and officials in Britain, the United States and other countries promised to tighten air cargo security.

Despite the heightened alert, in March someone shipped a hoax bomb -- which had a timer, wires and a detonator -- to Turkey via the UPS office in London.

Philip Baum, editor of Aviation Security International, said that since the toner cartridge plot "there has been a greater focus on air cargo security regimes, but that does not mean a great deal has changed."

But he said the UPS restrictions should be seen as positive.

"It means that problems have been found and are being rectified," he said.