WASHINGTON - Dramatic U.S. government test results raise new concern that bulk shipments of rechargeable lithium batteries carried as cargo on passenger planes are susceptible to fires or explosions that could destroy the airliners.

Yet U.S. and international officials have been slow to adopt safety restrictions that might affect the powerful industries that depend on the batteries and the airlines that profit from shipping them. The batteries are used in products ranging from cellphones and laptops to hybrid cars.

Shipments of rechargeable batteries on passenger planes are supposed to be limited to no more than a handful in a single box, under safety standards set by the U.N.'s International Civil Aviation Organization and adopted by the United States and other nations. But a loophole permits shippers to pack many small boxes into one shipment and get around the rules. Tens of thousands of the batteries may be packed into pallets or containers and loaded into the cargo holds of wide-body passenger planes.

In an April test by the Federal Aviation Administration, a cargo container was packed with 5,000 lithium-ion batteries and a cartridge heater added to simulate a single battery experiencing uncontrolled overheating. The heat from the cartridge triggered escalating overheating in nearby batteries, which spread in a chain reaction. Temperatures reached about 1,100 degrees Fahrenheit.

Once about 300 batteries had become involved, a fierce explosion blew open the container door and sent boxes flying, catching FAA and industry observers by surprise. Within seconds, the cargo container was in flames. The explosion came from a buildup of flammable gases.

The FAA said Tuesday that it was continuing its battery testing and searching for ways to prevent fires and explosions.