Aviation regulators in London have issued a formal warning over the possible radioactive hazard from Japan's stricken Fukushima nuclear power plant.

London Volcanic Ash Advisory Centre (VAAC), the world's focal point for notifying the aviation industry of International Atomic Energy Agency warnings on the release of radioactive material, issued a message covering 10 regions.

The open-ended warning, issued at 2 p.m. AEDT on Wednesday, applies to airspace zones in Japan, Russia, China, the United States and South Korea.

VAAC came to prominence last year when it was at the centre of coordinating the response to the Iceland ash crisis.

Most carriers are still flying to Tokyo, but German airline Lufthansa Tuesday diverted flights from the capital to other Japanese airports.