In Tokyo, shops began rationing goods - milk, toilet paper, rice and water - as a run on bottled water coupled with delivery disruptions left shelves bare on Thursday.

The unusual sights of scarcity in one of the world's richest, most modern capitals came a day after city officials reported that radioactive iodine in Tokyo's tap water measured more than twice the level considered safe for babies.

Radioactive caesium, 1.8 times higher than the standard level, was found in a green leafy vegetable grown in a research facility in Tokyo, Kyodo news agency reported on Thursday.

Radiation contamination has been found in various vegetables near the nuclear power plant, but this was the first to be detected in a vegetable grown in the capital.

Despite the situation, the Japanese people have shown a high degree of calm "beyond imagination", said a Chinese national, surnamed Deng, who works in Sendai, Miyagi prefecture, the area closest to the quake epicenter.

"Radiation levels in the soil and air are higher than usual, but it will not pose health risks," she told China Daily.

Imports from Japan such as seafood, drinking water and rice have to undergo radiation tests at customs, said Han Chunli, spokeswoman for the Shanghai Entry-Exit Inspection and Quarantine Bureau.

In Hangzhou, capital of Zhejiang province, radiation detectors have been installed at the airport.

Wang Jian, director of the airport inspection and quarantine department in Qingdao, Shandong province, said goods arriving from Japan had to undergo radiation screening.

In Hong Kong, Under-Secretary for Food and Health Gabriel Leung said on Thursday that by 2 pm the Hong Kong Centre for Food Safety had tested 29 imported shipments from Japan during the previous 24 hours and none were found to contain unacceptable radiation levels. The shipments included seafood, vegetables, fruits and snacks.

AP, Reuters, AFP and Kane Wu contributed to this story.