The level of radioactive iodine in the sea off Japan's disaster-hit Fukushima nuclear plant has soared to its highest reading yet at 4,385 times the legal limit, the plant operator said Thursday.

The level of iodine-131, reported a few hundred metres (yards) south of its southern water outlet has risen in a series of tests since last week, carried out by plant operator the Tokyo Electric Power Company (TEPCO).

Previous readings there were 1,250 times the legal maximum on Friday, 1,850 times the limit on Saturday and 3,355 times the limit on Tuesday.

A 9.0 magnitude earthquake and tsunami on March 11 knocked out the cooling systems of the Fukushima plant's six reactors -- triggering explosions and fires, releasing radiation and sparking global fears of a widening disaster.

earlier related report
Low-level Japan radiation spreads to most of China
Beijing (AFP) March 31, 2011 - "Extremely low" levels of radiation from Japan's crippled nuclear power plant have spread to most Chinese provinces but remain far too low to be a health risk, China's government said.

The Ministry of Environmental Protection said in a notice late Wednesday that radiation was detected across the country's heavily populated eastern, northern and southern regions.

The ministry said on Monday that radioactive iodine was detected in a handful of provinces, but subsequent statements have tracked a steady widening of the affected areas.

However, the latest ministry notice repeated earlier assertions that the amount of radioactivity was only about one-thousandth of what a person would receive during a 2,000-kilometre (1,200-mile) air flight.

Japan's atomic crisis has caused concern in China, sparking earlier panic-buying of salt nationwide as consumers mistakenly believed that the iodine it contained could protect against radiation poisoning.

The government has also banned imports of several food products from Japan and stepped up checks at airports, seaports and other travel hubs amid fears of radiation contamination.

The Fukushima Daiichi Nuclear Power Plant was hammered by Japan's March 11 twin earthquake and tsunami disasters, and workers have struggled with the dangerous task of trying to bring radiation leaks under control.