The UK Environment Agency said it would prosecute those responsible for poisoning a river in central England with deadly cyanide and killing thousands of fish in the process.

Officials have been pumping oxygen into the River Trent between Stoke-on-Trent and Yoxall after being alerted to the pollution.

"This incident is under control -- river water quality is improving," said Environment Manager David Lowe.

"Levels of pollutant in the river have fallen significantly, but we continue to monitor the situation closely. As a precaution, people and animals should stay out of the water until further notice."

Lowe said public drinking supplies had not been affected.

"But thousands of fish have been killed and the Environment Agency considers this to be a serious pollution incident," he said. "We will thoroughly investigate and seek to prosecute those responsible."

Severn Trent Water confirmed that cyanide had entered one of its sewage treatment works in Stoke, disrupting the treatment process and allowing partially treated sewage to be released into the river.

"I can confirm that our company is not linked to the disposal of the chemicals," said Simon Cocks, Waste Water Services Director for Severn Trent Water.

"Engineers have been working day and night to get the works back up and running. We are deeply concerned about the impact this chemical pollution has had on our sewage treatment system and the community in which we operate."

The Environment Agency said those found guilty of allowing poisonous or polluting matters into rivers faced unlimited fines. The largest ever penalty was a 240,000 pound fine levied against a chemical company in 2004.