Global investors fear that China's market turmoil will destabilise the financial system, and be a bigger risk than the crisis in Greece.
Japanese firms have shut factories in China and urged expatriate workers to stay indoors in the wake of angry protests over a territorial dispute.
Japan on Thursday said it would ban people entering the 20km evacuation zone around the Fukushima Daiichi nuclear plant north of Tokyo.
Analysts suggest power blackouts will ultimately cause the biggest economic damage to Japan, a vital player in the global manufacturing supply chain.
Plutonium in soil at the Fukushima nuclear complex heightened alarm on Tuesday over the battle to contain the world's worst atomic crisis in 25 years.
Engineers had to pull out of some parts of Japan's crippled nuclear plant on Thursday when three workers were exposed to high contamination.
Japan estimated the cost of the damage from its devastating earthquake and tsunami could top $300-billion.
The United States became the first nation to block produce from ally Japan's radiation zone, saying it will halt milk, vegetable and fruit imports.
Smoke and steam rose from two of the most threatening reactors at Japan's quake-crippled nuclear plant on Tuesday.
The WHO said on Monday that radiation in food after an earthquake damaged a Japanese nuclear plant was more serious than previously thought.