Thousands of planes will be checked

Officials ordered the searches to be completed within 24 hours but said there was no indication of a terrorist threat.

The mass action follows the discovery of plastic bags containing box cutters in lavatories on two Southwest Airlines planes, one in New Orleans, the other in Houston.

Box cutters were used by attackers who hijacked four planes on 11 September, 2001, and crashed them deliberately in New York and Washington killing more than 3,000 people.

An investigation into how and why the items got on the two planes has been launched by the Federal Bureau of Investigation (FBI) and other government agencies.

FBI Director Robert Mueller refused to give any details of the inquiry. But he said: "It does not appear to be a terrorist event and there is no immediate threat."

Banned items

Southwest Airlines said notes found with both packages said the intention was to simulate a threat and to challenge the security screening procedures for passenger planes.

Restrictions on carry-on items were tightened after 11 September 2001

The company said its employees found the potential weapons on Thursday evening. It said subsequent inspections of its entire fleet of 385 aircraft had found no further suspicious items.

Box cutters and other types of knives and items that could be used as weapons are banned from passengers' carry-on bags.

Bleach and some form of clay made to resemble plastic explosive were also reported to have been in the packages.

More than 5,000 passenger jets which make about 23,000 flights daily in the US will be searched.