The Welsh Ambulance Service says it was called to a crash on the A465 eastbound, between Aberdulais and Resolven, at 3.42pm.
A lorry and a van have gone down an embankment.
There is no news on casualties yet, as they are still on the scene.
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Feeling humid with heavy showers and thunderstorms moving north overnight into Sunday.
Four people, including three children, were taken to hospital to be checked for smoke inhalation.
Heavy rain and thunderstorms will affect many parts of Wales this weekend - particularly during Saturday evening.