LONDON - U.S.-based shipping firm UPS has been ordered to stop moving air cargo through some of its U.K. facilities because of security flaws, the British government said Friday.

The order is the result of a planned security check rather than a new threat to aviation - and a sign of heightened concerns about the vulnerability of cargo in the wake of an al-Qaida plot that saw bombs disguised in toner cartridges shipped on freight flights from Yemen.

Britain's Department for Transport said that "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."

It gave no details of the security issues and didn't identify the locations involved. No other air freight companies were mentioned in the statement.

Atlanta-based UPS said the restrictions were the result of an ongoing security review rather than any specific threat.

UPS spokeswoman Susan Rosenberg said six of the company's British package-screening facilities were still operating, its main air hub at East Midlands airport in central England was open and its planes were still flying to and from Britain.

She would not say how many facilities had been shut down, how many packages were being disrupted or whether packages being sent to Britain as well as those leaving the country were affected, citing the need to preserve secrecy around security arrangements.

The U.S. has not taken similar measures, because UPS Ltd. UK is a separate subsidiary of UPS and is not regulated by the U.S. government, an American Transportation Security official said. The official said the U.S. already has "stringent security protocols" in place for air cargo security.

UPS told customers Friday that some 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."

The company said some facilities had temporarily been "taken offline," leading to delays in package movements.

"UPS has activated contingency plans, communicated with customers and expects service levels to return to normal early next week," the company said. "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."

International aviation security has been an urgent priority since the Sept. 11, 2001 attacks, and 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 (160 kilometers) north of London.

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

Countries including the United States, Britain and Germany - which the bombs had passed through - suspended all unaccompanied air cargo from Yemen and Somalia after the thwarted plot, and promised to tighten air cargo security.

Overall security rules, however, still vary from country to country, and between cargo-only flights and passenger flights.

Even before 9/11, cargo-screening rules for passenger flights were tightened after airplane bombings in the 1970s and the 1988 bombing of Pan Am Flight 103 over Lockerbie, Scotland, that killed 270 people.

The United States requires screening of all packages aboard passenger flights originating in the country - but there's no such requirement for cargo on U.S.-bound passenger flights or on cargo-only flights such as UPS or FedEX planes.

Proposals to screen all packages on cargo flights have long been opposed by the freight industry, which argues that slowing down shipping for inspections would jeopardize the industry and the world's economy.

Despite the heightened state of alert after last year's bomb plot, 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 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."

"In fact, very little has changed," he said.

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

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