A long stretch of river containing toxins from a chemical plant explosion in China has reached the northeastern city of Harbin. Water in the city of nearly four million people has been turned off for two days as authorities wait for high concentrations of benzene to dilute.

Residents stand on the banks of the Songhua River in Harbin, Thursday

China's central government on Thursday worked to explain why officials waited 10 days before informing people in Harbin that the blast several kilometers up the Songhua River had dumped dangerous amounts of poisonous, cancer-causing benzene into the waterway. The river is Harbin's main source of water.

Vice Environmental Minister Zhang Lijun told reporters in Beijing Thursday the company, a subsidiary of a state-owned petroleum company, should be held responsible for the spill.

When asked why officials delayed telling the public, he defended the provincial authorities' decision to wait.

Mr. Zhang said the provincial government's methods in releasing information were practical and insisted the public was not affected.

Officials are waiting for the contaminated section of the partly frozen river to pass all of Harbin's intakes. They estimate that could happen by Saturday.

Little information was available from villages upstream, where some residents earlier reported seeing many dead fish.

Farmer protests over pollution have become more common in China in recent years, and in some cases have been violent. Observers said the government appears eager in this case to stem any possibility of unrest.

Environmentalist Dai Qing, a vocal critic of the government's policies in the past, calls the response this time unprecedented. She said the fact that officials admitted - albeit 10 days later - that a toxic spill had occurred is an improvement.

"Starting last year, environment protection in China has been improving, I think, for two reasons: First, Hu Jintao and his "protect people first" policy seems to show he wants to change the path that previous president Jiang Zemin was on, which was benefit people with power and money," he said.

The Songhua river flows into another waterway that follows part of China's border with Russia - prompting Beijing to offer repeated reassurances to Moscow that it is monitoring toxin levels. Mr. Zhang on Thursday said Chinese and Russian officials were talking about setting up a hotline to keep each other informed.

In Harbin Thursday businesses remained open and stores restocked bottled water, having run out earlier in the week when the government first announced the taps would be turned off.