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.
NRW said it found the source of the pollution at 19:30 BST on Friday but will not be naming it at this stage, and its officers are working to identify how it entered the drainage system.
Kerosene has now spread down almost 2.5 miles (4km) of the river, it added.
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”.
Reports of a leak affecting one mile (1.6km) of the river were first received on Saturday morning 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 Taff’s Well, Rhondda Cyon Taff, said: “It smelt like a petrol station, it hit you as soon as you came out.”
Absorbent materials have been deployed to try and contain the leak.
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”.
“NRW have told us that up to 25,000 litres of kerosene has entered the river.”
Gareth Llewelyn, a former hazardous materials environmental protection officer with South Wales Fire and Rescue service, said there was “no quick fix” and it needed “years not months” to restore.
He added: “I have visited the culvert across the road from the GE site yesterday and it was full of kerosene. It doesn’t mix with water and will continue to pollute.”
Llewellyn said the spill has “torn the heart out of our community”, adding: “The village is Taff’s Well – it’s named after the river.”
He said “now there’s no animals, no birds” and all the fish have died.
