Nearly 30 elementary school children in the Philippines have died after eating fried cassava balls obtained from a vendor, a local official told CNN.

Fourteen children died Wednesday before reaching hospitals and 13 died upon arrival, a spokesman for Ubay Mayor Eutiquio Bernales said.

Two more children also have died, but it was not clear when. Thirty-five more are in critical condition.

The victims suffered severe stomach pain, then vomiting and diarrhea after eating the snack during morning recess, The Associated Press reported.

They were taken to at least four hospitals near the school in Mabini, a town on Bohol Island, about 610 kilometers (380 miles) southeast of Manila, AP said.

"Some said they took only two bites because it tasted bitter and the effects were felt 5-10 minutes later," Dr. Harold Gallego of Garcia Memorial Provincial Hospital in the nearby town of Talibon told AP.

The vendor who sold the cassava balls insisted nothing was wrong with them and ate a few to prove the point. Now she, too, is in critical condition.

All of the children were in the first and second grades of San Jose Elementary School in Mabini. Francisca Doliente AP that her 9-year-old niece Arve Tamor was given some of the deep-fried caramelized cassava by a classmate who bought it from a regular vendor outside the San Jose school.

"Her friend is gone. She died," Doliente said, adding that her niece was undergoing treatment.

Grace Vallente, 26, said her 7-year-old nephew Noel died en route to the hospital and that her 9-year-old niece Roselle was undergoing treatment.

"There are many parents here," AP quoted her as saying from L.G. Cotamura Community Hospital in Ubay.

"The kids who died are lined up on beds. Everybody's grief-stricken."

Cassava is a starchy, tuberous root that is a low-cost source of carbohydrates in humid, tropical areas; it is also the source of tapioca.

Cassava contains amino acid-derived cyanogenic glucosides -- some more than others -- and must be thoroughly cooked to remove toxic levels.

Eaten raw, the human digestive system will convert part of it into cyanide. Two cassava roots contain enough to be fatal.

Source: Agencies