Japan's volatile Fukoshima prefecture, the centre of its ongoing nuclear crisis, and three other prefectures have been ordered to halt some food shipments over fears of radioactive contamination.

The news comes on the same day the World Health Organization said radiation levels in some food products are more serious than first thought.

Japan's Kyodo News service reported Monday that the order covered spinach and kakina, another leaf vegetable product.

The order was issued in Fukoshima, Ibaraki, Tochigi and Gunma prefectures after radioactive substances were detected at levels above the legal limit.

Trace mounts have also been found in tap water samples taken in nine prefectures. Fukoshima -- the source of the radioactive material leaking from the troubled Dai-ichi power plant -- was asked to stop shipping raw milk.

Still, the government stressed the order was a precautionary measure. Top government spokesperson Yukio Edano said the levels detected "aren't readings that would affect humans."

That message was echoed by Chief Cabinet Secretary Edano, who urged the public not to panic.

"Eating food with (radioactive levels) exceeding provisional limits isn't going to affect your health," he said.

Peter Cordingley, a spokesperson for the World Health Organization, said the situation was "serious" in an interview with Reuters. However, he said there is no evidence that contaminated products got beyond Japan's borders.

China and South Korea later announced plans to implement tougher checks of Japanese food for radioactivity.

In China, the Xinhua News Agency reported that the nation's national quality watchdog would be monitoring food from Japan.

In South Korea, officials will broaden radiation inspections to include dried agricultural and processed food made from fresh agricultural produce.

According to Xinhua, China imported $593 million worth of food products from Japan in 2010.