BBC Wales's Geraint Vincent reports on the second incident of pollution in less than a month

It is thought the fish were killed after a large amount of silt and cement seeped into the River Llynfi near Maesteg on Tuesday night.

It is the second serious pollution incident in south Wales in less than a month.

Mainly young trout and salmon died after being suffocated by sediment which had seeped into the river from a nearby building site.

This pollution comes just three weeks after more than 5,000 fish were killed in a similar incident on the River Tawe near Ystradgynlais in the Swansea Valleys.

The Environment Agency's Paul Edmonds

The Environment Agency has described the situation as "very worrying".

"We are very concerned about this spate of incidents and we would urge industries, construction sites, farmers and the like who store oil chemicals and who are working near rivers, to take adequate precautions," said Paul Edmonds of the Environment Agency.

Steps are now being taken to stop the silt from spreading further downstream.