The road section of the only crossing from occupied Crimea has collapsed after an explosion.
Widely shared images show a train on the Kerch rail bridge ablaze, with the adjacent road bridge appearing to have sustained damage.
Russian investigators have reported three deaths.
The bridge, built on the orders of Vladimir Putin, is a key link to move military equipment into Ukraine. Moscow had illegally annexed the region four years earlier.
Read more: Crimea bridge partly reopens after blast – Russia