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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Runners looking to compete in the Newport half marathon have been disappointed again after it was cancelled due to weather warnings.
Very cold and windy. Some sunny breaks but plenty of snow showers feeding in the from the east.