Zhang Lijun, vice head of the State Environmental Protection Administration, delivered the somber message as he warned of the danger of the substances currently being carried along the icy waters of the Songhua River.

"Nitrobenzene is very harmful for the environment," Zhang told reporters at a briefing in Beijing, called amid the escalating environmental scare in the two provinces of Jilin and Heilongjiang.

"To address this issue, the government has ordered the environmental bureaux of the two provinces to continue to monitor the impact of the nitrobenzene to the environment even after the pollution has been cleaned up," he said.

He cited experts as saying 100 tonnes of benzene and benzene derivatives were released into the river after a massive explosion at a PetroChina chemical plant in Jilin province on November 13.

As a result, an 80-kilometer (50-mile) chemical slick has been slowly moving downstream along the Songhua river and reached Harbin, a city of nearly four million people, on Thursday morning.

Benzene is a carcinogen that can be lethal to humans, even in small doses, while nitrobenzene is known to create blood disorders.

The government had said on Wednesday that the toxicity in the Songhua river had exceeded national safety levels by more than 100 times near the blast site.

However Zhang did not say how the toxic slick in the Songhua may specifically effect humans or the local environment.

An expert from the Chinese Research Academy of Environmental Sciences, Yu Yingjiang, was equally vague.

"Benzene compounds are definitely harmful, but researchers at present still haven't figured out how harmful it is," Yu said.

Zhang said it was too early to talk about criminal offenses in connection with the blast, but said the responsibility of the accident lay squarely with PetroChina, the oil major in charge of the chemical plant.

"We're very clear about the responsibility," he said. "It's the responsibility of the PetroChina petrochemical complex in Jilin."

All rights reserved. � 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.