An investigation is ongoing to find the source of pollution after hundreds of fish were found dead in a Powys river.

Natural Resources Wales officers have been on site collecting evidence at Afon Llynfi in Pontithel, near Glasbury for a third-consecutive day on Monday (August 3).

"This investigation is a top priority for us and we are putting all the appropriate resources and expert staff in place to investigate it fully," a Natural Resources Wales spokesperson said.

READ MORE: Natural Resources Wales investigate pollution incident as 200 fish are found dead

County Times: The Llynfi is a Site of Special Scientific Interest due to the local population of white-clawed crayfish. Picture: Natural Resources WalesThe Llynfi is a Site of Special Scientific Interest due to the local population of white-clawed crayfish. Picture: Natural Resources Wales

"The incident has had a devastating effect on the Afon Llynfi as the pictures taken over the weekend show. The Llynfi is a Site of Special Scientific Interest due to the local population of white-clawed crayfish.

"While we believe that the pollution has stopped entering the river, the investigation is ongoing, and we are committed to finding the source of the pollution."

Officers from Natural Resources Wales found around 200 dead bullhead fish in a seven-metre stretch of the Llynfi which is a tributary to the River Wye.

Two officers are currently undertaking a fish-kill assessment and another two officers are also on site continuing with the investigation into the source of the pollution.