South Wales Police says the A465 is closed between Aberdulais and Resolven, after a serious crash.
Diversions are in place via the B4242.
More top news
All primary and secondary schools in Wales have been categorised in a new colour-coded system. Is your school green or red?
A yellow warning for snow and ice on Thursday night and Friday has been issued.
Some wintry showers and staying windy with a significant wind chill and widespread frost developing