Schools reopened in northeast China’s Harbin Tuesday after a toxic slick had passed by the city along the Songhua river, as the foreign ministry vowed to “go all out” to protect Russians living downstream.

A total of 400,000 primary and secondary school students returned to classes, as safety measures were kept in place to ensure they did not drink poisoned water, Xinhua news agency reported.

The city’s education bureau had requested schools to buy quality drinking water for the students or tell the children to bring bottled water from home, according to Xinhua.

Harbin suffered a five-day shutdown of water supplies after an explosion at a chemical plant upstream released 100 tonnes of benzene and nitrobenzene into the Songhua river, which provides most of the city’s drinking water.

Fifty tonnes of the chemicals were estimated to have been absorbed in the river bed and deposited along the banks above the city, Xinhua said, citing environment officials.

The other 50 tonnes were believed to have passed through Harbin and were still posing a danger for many others in cities downstream, and also on the Russian side of the border.

Zhang Lijun, vice director of China’s State Environmental Protection Administration, held talks late Monday with a delegation from Khabarovsk, the part of Russia that will be impacted the most by the slick.

Zhang apologized on behalf of the Chinese government for the impact the pollution would have on Russia, the People’s Daily reported on its website.

He also briefed the delegation in detail about the river pollution, describing the sorts of pollutants the Russians could expect to see.

“We will go all out to reduce the possible harm of the Songhua pollution to the Russian side,” foreign ministry spokesman Liu Jianchao told a regular briefing.

“We will do whatever it takes to ensure the safety of Chinese and Russians,” he said.

He said China had always taken a responsible attitude on the issue of cross-border rivers, adopting measures that gave “full consideration for the concern of downstream countries”.

“Neither China nor Russia wishes to see harm to our relations from this incident,” Liu said.