South Wales Police says the A465 is closed between Aberdulais and Resolven, after a serious crash.
Diversions are in place via the B4242.
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A dry night with clear spells in the west. Temperatures will fall, with thickening cloud inland by the early hours.
The First Minister Carwyn Jones is under fire after claiming that the Tata Steel management buy-out team "has not yet raised the money".
Police say they want "no surprises" from Welsh football fans.