People in Crimea have been told to avoid using the bridge connecting the occupied peninsula with Russia, after what officials called an "emergency".
They did not call it an attack, but unconfirmed reports said that locals heard explosions early on Monday.
The Kerch bridge was opened in 2018 and enables travel between Russia and Crimea - Ukrainian territory held by Moscow's forces since 2014.
In October last year, it was partially closed following a major explosion.
The bridge - which is an important supply route - fully reopened in February.
Writing on Telegram, Sergey Aksyonov, the head of the Russian administration in Crimea, said: "Traffic was stopped on the Crimean bridge. An emergency occurred in the area of the 145th support from the [Russia side of the bridge].
"Law enforcement agencies and all responsible services are working. Measures are being taken to restore the situation.
"Given the current situation, I ask residents and guests of the peninsula to refrain from traveling through the Crimean bridge and, for security reasons, choose an alternative land route through new regions."
This is how Russia refers to the Ukrainian territories it occupied last year and claims to have annexed.
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