There are fears for wildlife after a train carrying up to 750 tonnes of diesel derailed, causing an oil spill at a scientifically important site.
The freight train derailed and caught fire in Llangennech, Carmarthenshire, on Wednesday night.
Ten wagons, each containing up to 75 tonnes of diesel, derailed, with oil spilling into the Loughor Estuary.
Natural Resources Wales (NRW) was not able to investigate the impact until the fire was put out.
NRW officers are now on site to advise on the containment and recovery of the diesel to prevent any further pollution of the estuary.
The site of special scientific interest is home to wildfowl and wading birds.
Ben Wilson, NRW’s incident manager, said: “We also have monitoring teams out assessing the extent of the pollution.
“Initial modelling suggests that oil pollution is currently restricted to the upper reaches of the estuary around Loughor Bridge. This is supported by observations on site. We’ve also seen significant amounts of oil three miles downstream from here.