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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Rail users in Wales could be hit by disruption later this week after talks to avert a 48-hour strike broke down.
Prince Charles was all smiles during his visit to Cardiff today meeting cricketers and enjoying a chat over a cup of tea.
The Prince of Wales met members of the England and Australia cricket teams ahead of the Ashes tournament which starts on Wednesday.