Lufthansa scanning planes out of Japan for radioactivity, but not Qantas/Jetstar

German international airline Lufthansa says it has started scanning planes ex-Japan for signs of radioactivity, as the nuclear crisis at four power plants worsens.

Japan's Prime Minister, Naoto Kan, has warned people up to 30KM from one of the affected power plants to stay inside, with chief government spokesman Yukio Edano confirming that radiation leakage from the plants now poses a risk to human health.

"There is no doubt that unlike in the past, the figures are the level at which human health can be affected," he said.

Lufthansa says it has not detected any radioactivity from its planes, but is taking the step as a precaution to avoid endangering passengers.

However, it has withdrawn its flagship Airbus A380 from flights to Japan, and is also accommodating flight crews overnight in Korea rather than Japan.

Air China has also cancelled flights to Tokyo today and tomorrow, saying it does not want its aircraft staying on the ground in Tokyo overnight.

Jetstar, which is still operating flights in and out of Japan, said it was not specifically scanning planes for signs of radioactivity.

"From our perspective, we are continuing safe operations to and from Japan and closely monitoring the situation," a Jetstar spokesperson told Australian Business Traveller.

Qantas said it was not required to scan its planes for radioactivity.

"It is European policy for aircraft to be tested. We are monitoring the situation very closely and meeting all Australian Government requirements. Should this change, we will continue to meet all government requirements," spokesman Simon Rushton said.

According to Reuters, other airlines that have no concerns about radioactive contamination include Cathay Pacific, AirAsia, Singapore Airlines, Thai Airways, American Airlines and Delta.