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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None of the people onboard the boat had been wearing a lifejacket. They were spotted clinging to the hull of their upturned boat.