No cases of tainted water or milk have been reported in China but the ministry vowed to further strengthen monitoring.

Wang Zhongwen, a researcher at the China Institute of Atomic Energy's radiation safety department, told China Daily on Wednesday that currently China only had the means to conduct food radiation tests in a few regions.

The statement also said that recent rain in Beijing and Tianjin meant radioactive substances could have fallen on vegetables.

Chen Jicang, a vegetable dealer in Beijing, said on Wednesday that concerns are growing among consumers, businesses and governments across the world.

"We have yet to see any impact on our business from the radiation, but we will keep a close eye on how the issue develops," he said.

He added that most vegetables consumed in Beijing are produced locally, and he might switch to other varieties if spinach sales suffer.

Yang Guoshan, a researcher of radiation medical science at the Academy of Military Medical Sciences, said people could safely eat products that contain low levels of Iodine-131.

"The radiation level is so low that residents don't need to spend any extra effort on cleaning them," he said.

According to the World Trade Organization in 2006, Yang said, products with an iodine level up to 100Bq/kg are safe.

Gao Jie, a 52-year-old resident in Tianjin said she is deeply worried because spinach is a staple part of her family's diet.

"If spinach is radioactive, is there any possibility that other types of vegetables are safe?"

Trace levels of radioactive isotope cesium-137 and -134 were detected in the air in 21 provinces and regions on Wednesday, up from 17 on Tuesday, according to China's National Nuclear Emergency Coordination Committee.