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A large fire erupted on the rail bridge between the Crimea peninsula and Russia in the early hours of Saturday.
Russian state media says a fuel tank is on fire, without specifying a cause.
Footage appeared to show a train on the Kerch Strait rail bridge ablaze, with the adjacent road bridge also appearing to have sustained damage.
Crimea was annexed in 2014 by Moscow, which now uses the bridge to move military equipment into Ukraine.
An adviser to the head of Crimea, quoted by the Russian state agency Ria Novosti, said work is under way to extinguish the fire.
Russian state media, meanwhile, said traffic has been suspended on the bridge.
The Crimean bridge is said to be a key target for Ukrainian forces who have been attacking Russian logistics.
The bridge across the Kerch Strait was opened by President Vladimir Putin in 2018, four years after Moscow illegally annexed Crimea.
The crossing is more than 100 miles from Ukrainian-held territory. One explosives expert told the BBC said the fire was probably not caused by a missile.
"The lack of obvious blast / fragmentation damage on the road surface suggests that an air-delivered weapon was not used," he said
He said it was possible that "a well-planned attack from below may have been the cause".