The affected prefectures are Fukushima, Ibaraki, Tochigi and Gunma, a health ministry official told AFP.
Prime Minister Naoto Kan told prefectural governors to halt shipments of broccoli and "komatsuna" green leaf vegetables from Fukushima, and untreated milk and parsley from neighbouring Ibaraki, media said.
Public broadcaster NHK said Kan had also told people in those areas not to eat the vegetables, which were found to have abnormal radiation levels because of releases from the nearby crippled nuclear power plant.
Radioactivity drastically exceeding legal limits set under Japan's food sanitation law had been found in 11 kinds of vegetable grown in Fukushima, also including cabbage and some green leaf vegetables, the health ministry said.
Radioactive caesium at 82,000 becquerels 164 times the legal limit was found in one type of leaf vegetable, along with 15,000 becquerels of iodine, more than seven times the limit, the ministry said.
The ministry said that if people eat 100 grams (four ounces) a day of the vegetable for about 10 days, they would ingest half the amount of radiation typically received from the natural environment in a year.
Officials in six more prefectures Miyagi, Yamagata, Niigata, Nagano, Saitama and Chiba, the last two bordering Tokyo have been asked to step up radiation monitoring of farm products, the health ministry official said.
Even if the short term risk is limited for now, scientists pointing to the 1986 Chernobyl disaster warn that some radioactive particles stay in the environment for decades and concentrate as they travel up the food chain.