More than 900 pupils at elementary schools in a central Japanese city have fallen ill in a suspected outbreak of mass food poisoning, officials said Thursday.

The number of students who reported vomiting and diarrhoea reached 905 by late Thursday at 14 elementary schools in Hamamatsu, a seaside city around 200 kilometres (125 miles) southwest of Tokyo, the local education board said.

About 40 teachers and staff at these schools also showed symptoms of food poisoning, said the board.

"Most of them developed such symptoms last night. They had eaten school lunch but any causal relation with the symptoms have yet to be confirmed," Seiko Terada, director of health and school lunch at the board, told AFP.

The affected schools were closed Thursday and will remain shut until at least the end of Friday.

"We have detected norovirus in five stool samples taken from some of the affected pupils," Hamamatsu city health official Toshihiko Furuta said.

The incident follows reports of more than 1,400 people falling sick across Japan after eating frozen food suspected to have been laced with pesticides.