US airlines are beginning to operate again this evening on a limited basis as more stringent levels of security are implemented.

Transatlantic flights from Ireland remain cancelled with no firm decision yet as to when they may resume.

British Airways, Europe's biggest airline, said their U.S. flights would also remain cancelled pending details on new US security arrangements.

US Transportation Secretary, Norman Mineta, said it would be a colossal task to switch back on the American civil air transport system, which carries 1.6 million people on a normal day with 36,000 to 40,000 departures.

"We will reopen airports and resume flights on a case-by-case basis, only after they implement our more stringent levels of security," the US transport chief said.

The new rules include a total ban on passengers carrying aboard any cutting implements such as scissors or knives of any material or size. Previously, only knives with blades longer than four inches were barred.

There will be a thorough search and security check of all airplanes and airports before passengers are allowed to board.

Other regulations ban baggage check-ins at any place besides the airport ticket counters and only passengers with tickets will be allowed to proceed past airport screeners to catch their flights.

Meanwhile, Britain's temporary ban on planes flying low over central London is to be lifted from midnight on Saturday.

The small City Airport in east London near the capital's financial district is to be re-opened tomorrow.

The ban had meant no planes were allowed to fly over central London below a height of 6,000 feet. The restrictions also meant planes using Heathrow Airport have had to take different flight paths. Planes flying over but not landing in London, and operating above 6,000 feet, have not been affected.