One in four nursery school children caught up in Japan's 2011 tsunami disaster has psychiatric problems caused by the horrors of loss and destruction, with experts warning of a dire shortage of psychiatrists.

Researchers say that for some children, the effects may be felt throughout their lifetimes unless they get the help they so urgently need.

A study found 25.9 percent of children aged between three and five suffer from symptoms including vertigo, nausea and headaches, with some exhibiting worrying behaviour such as violence or withdrawal.

Youngsters were scarred by losing friends, seeing their homes destroyed, by separation from parents or by the sight of the huge wall of water that crashed ashore, the study team said.

The group, led by professor Shigeo Kure of Tohoku University School of Medicine, said young children who do not receive the necessary care could develop much worse problems in later life.

These could include developmental disorders and learning disabilities, which would affect academic achievement and employment prospects, "as they may have trouble in communicating with other people due to the influence of experiences related to the disaster", Kure told AFP.

More than 18,000 people died when a 9.0-magnitude undersea earthquake sent a towering tsunami into Japan's northeast coast in March 2011.

The country's worst post-World War II disaster was compounded by reactor meltdowns at the Fukushima nuclear power plant, which sent tens of thousands of people fleeing from radiation.

Researchers looked at 178 children whose parents or guardians agreed to cooperate in the three areas worst-hit by the catastrophe -- Iwate, Miyagi and Fukushima.

They used an internationally recognised child behaviour checklist and met children between September 2012 and June last year.

The level of children who need psychiatric care is up to three times that seen in parts of Japan unaffected by the disaster -- for example, 8.5 percent of children in Mie prefecture in central Japan need help.

"I was surprised at the percentage of children who need medical care. I didn't expect it would be this high," Kure said.

"These children who were part of our study have received and will keep receiving psychiatric care in the coming years, but another issue is how to make contact with children whose need for psychiatric care has not yet been identified," he said.

"I imagine there are lots of children, who seemingly are leading normal lives but show behaviour that needs a doctor's attention, for example, waking up suddenly at midnight or biting their nails."

What makes the situation more difficult is the scarcity of child psychiatrists in rural Japan, he continued.

Even in Sendai, the biggest city in the area with a population of more than a million, "there are only a handful of child psychiatrists as far as I know," he said.

"What child psychiatrists in our team have been doing is to ask parents and teachers to pay attention to children" due partly to the shortage of psychiatrists, he said.

Makiko Okuyama of the National Center for Child Health and Development, who took part in the study, said the results were worrying.

"It is known that children need (psychiatric) care right after an earthquake disaster, but this study was done more than a year and half after the fact, so that concerns me," she said, according to the Mainichi Shimbun.

An official at the health ministry said officials would closely examine the final report when it is produced in around two months' time and consider whether current psychiatric healthcare provision is meeting needs.