Local officials meanwhile entered the 20-kilometre no-go zone around the stricken Fukushima nuclear plant to assess whether the thousands of cattle and other livestock abandoned in the no-man's-land must be culled.

Radiation has leaked into the air, soil and ocean from the plant, leading to bans on some regional farm produce -- the latest on shiitake mushrooms from a nearby town that were found to have radiation levels twice the legal limit.

The disaster's political and economic shockwaves kept rippling through Japan, as Prime Minister Naoto Kan came under renewed pressure over his crisis management and automakers reported huge production shortfalls for March.

Toyota, the world's biggest automaker, said March output in Japan plunged 62.7 percent year-on-year, while Nissan suffered a 52.4 percent domestic decline and Honda Motor a 62.9 percent fall.

AFP