The containers, left in the city of Qiqihar by Japanese troops at the end of the war 58 years ago, were sealed with lead and wrapped in plastic, Xinhua news agency reported.

The team of experts was dispatched from Japan after the containers were discovered by construction workers early this month, and one of them leaked an oil-like substance that proved potentially fatal.

Initially, 35 people were hospitalized, two of them deemed in a critical condition, and new victims continue to appear, according to Xinhua.

The risk of contagion has increased because polluted soil from the construction site where the barrels were found was later removed to several different places in the city.

Gai Zunxu, a primary school student, developed blisters on the backs of his feet this week after he played on top of soil contaminated with the substance, the agency said.

Doctors at the No. 203 Military Hospital, which treats the patients, warned in the China Daily newspaper that more victims could emerge in the days ahead, because symptoms of mustard gas can take up to one month to appear.

Qiqihar, which was occupied by the Imperial Japanese Army for 14 years, still finds ghastly reminders of the war on a regular basis.

Since 2001, a total of 775 bombs and artillery shells and 28 gas containers have been discovered in the city, according to Xinhua.

It is estimated that more than 700,000 chemical weapons were abandoned throughout China by Japanese soldiers in the closing months of World War II.

For this and other reasons, Japan's brutal occupation of Chinese territory before and during the war remains a source of tension between the countries.

China this week expressed "solemn representations" over the chemical weapons find in Qiqihar.

On Friday, the Chinese foreign ministry reacted angrily after four Japanese cabinet members marked the 58th anniversary of their country's surrender with a visit to a controversial Tokyo war shrine.

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