In Vietnam, hand, foot and mouth disease outbreak has claimed 81 children, said health officials.
About three-quarters of the more than 30,000 reported HFMD cases so far this year have occurred in children younger than three, said Graham Harrison, the World Health Organisation's acting representative for Vietnam.
"It's a challenge to deal with it," he told AFP. "It's certainly a significant increase on the previous two or three years."
All of the dead were children, Vietnam's health ministry said.
HFMD is a common viral illness of infants and children that causes fever and blister-like eruptions in the mouth, according to the US-based Centers for Disease Control and Prevention, which are working with the WHO and Vietnam's health ministry to control the outbreak.
Harrison said the reason for the rise in HFMD cases this year is unclear and could not explain why almost all of them have been detected in the south of the country.
The severity of outbreaks tends to be cyclical, and other Asian countries have reported similar problems over the past decade, he added.
There is no specific treatment for HFMD, which is commonly spread through unwashed hands or contact with virus-contaminated surfaces.
Harrison said the outbreak highlights the need for good hygiene standards that include washing hands and disinfecting floors.
"I think it's fair to say the Ministry of Health has taken this quite seriously," he said.
Health Minister Nguyen Thi Kim Tien on Monday urged greater public education about HFMD and how to protect children, the official Vietnam News Agency said.
"We are trying to keep the death rate as low as possible," she said.
Vietnamese officials are debating whether to declare an epidemic.