TOKYO, Nov 28 (Reuters) - Japan vowed on Friday to conduct stringent surveys after initial studies found that poison gas left over from World War Two may have been dumped in more than 100 sites throughout the country.

Japan has long been under fire over chemical weapons it abandoned in China after the war, but concern has risen that compounds used in such weapons may also pose a threat at home after several recent incidents in which people became ill.

An official at the Environment Ministry said a survey it carried out in June found 138 suspected chemical weapons disposal sites in 41 of the nation's 47 prefectures.

"We will conduct further surveys in areas where this is needed," the official said.

The survey took place after residents of Kamisu, a town slightly north of Tokyo, complained of health problems and tests of their well water yielded levels of arsenic 450 times higher than mandated by government safety standards.

The contamination was believed linked to chemicals produced by the wartime military for use as weapons.

The discovery of the contamination prompted the government to offer Kamisu residents financial support, and it is now paying medical costs for around 80 people.

According to a 1973 government survey made public in April, Japan disposed of 3,875 tonnes of poison gas after World War Two.

A year ago, several construction workers fell ill after being exposed to mustard and tear gas agents stored in beer bottles unearthed at a construction site.

Japan has been criticised, and faced lawsuits, over its abandonment of an estimated 700,000 chemical weapons in China.

In October, China said Japan had agreed to pay 300 million yen ($2.75 million) to help resolve a row over injuries caused by such weapons. ($1=109.11 yen)