People have been advised to keep their windows and doors shut due to a widespread fuel leak in the River Taff.
Natural Resources Wales (NRW) said it had received multiple reports about pollution affecting the Taff in places such as Nantgarw, Treforest and Taffs Well.
GE Aerospace confirmed fuel got into the river while repair work took place at its site nearby.
NRW said the source was identified at 19:30 BST on Friday, and its officers are working to identify how it entered the drainage system, after kerosene spread down almost 2.5 miles (4km) of the river.
“The safety of our employees, neighbours, and environment is our top priority,” GE Aerospace said in a statement.
“We are working closely with Natural Resources Wales and South Wales Fire and Rescue Service to clean up a fuel spill that occurred on site while repair work was under way.”
The environmental body also received reports of pollution affecting the Taff in Glan Y Llyn, Gwaelod-y-garth and Tongwynlais.
It said South Wales Fire and Rescue Service workers helped with specialist equipment to reduce the amount of kerosene entering the river.
“Our officers are again on site where action has been taken to significantly reduce the amount of pollution entering the tributary, whilst investigate work is ongoing to try and stop the pollution entirely,” NRW said in a statement.
It is also taking samples to understand “the actual environmental impact of the spill”.
In an update on Sunday evening, NRW said due to the nature of the incident, identifying how the fuel entered the drainage system is difficult as it involved underground systems.
Specialist drainage and containment contractors have been sent to the site and are removing the fuel.
“Due to high flows and the width of the channel, the River Taff cannot not be ‘boomed’ with fuel absorbent barriers,” it said.
“Instead, a large quantity of fuel absorbents has been placed at the confluence between the impacted tributary and River Taff.”
However, action on Friday and throughout the weekend has significantly reduced the amount of fuel entering the river, NRW added.
It also said a number of dead fish have now been recovered from the river to be analysed and kept for evidence.
Reports of a leak affecting one mile (1.6km) of the river were first received on Friday from residents.
NRW later said it had spread as it continued “to investigate the source of the incident to understand how it is happening, stop the pollution completely and collect evidence”.
Eurig Thomas, a resident in Nantgarw, Rhondda Cyon Taff, said: “It smelt like a petrol station, it hit you as soon as you came out.”
Thomas added: “The river had a shiny sheen on it, the stream flows behind my house, it comes down from the GE factory and into the Taff.
“The smell has dissipated – but you can still smell it this morning.”
Another resident, Sarah Llewelyn, added: “I haven’t slept in three days. Friday afternoon lunchtime I opened the door and could smell fumes, it was burning my eyes.”
She dubbed it “a national disaster”.
