A blast on the strategic road-and-rail bridge linking Russia and the Crimea peninsula on Saturday brought down sections of the road taking traffic in one direction, prompting gleeful messages from Ukrainian officials but no direct claim of responsibility.
Russia seized Crimea from Ukraine in 2014 and the 19-km (12-mile) bridge linking it to Russia’s transport network was opened with great fanfare by President Vladimir Putin four years later.
It now represents a crucial supply route for the Russian forces who have taken control of most of southern Ukraine's Kherson region, reports Reuters.
The Russian National Anti-Terrorism Committee said on social media that the blast occurred at 6:07 am (0307 GMT) in a freight truck, and caused seven fuel tanker wagons to catch fire on a train heading for the peninsula.
It said two sections of road bridge had partially collapsed, but that the arch spanning the Kerch Strait, the waterway through which ships travel between the Black Sea and Azov Sea, was not damaged.
Sergei Aksyonov, the Russian governor of Crimea, said on social media that the road bridge was still intact in one direction, although traffic was suspended while the damage was assessed.
Images showed thick smoke pouring from part of the bridge.
Later, the Emergency Ministry said the fire had been extinguished, the Russian news agency Interfax reported.
A Ukrainian presidential adviser posted a message on Twitter saying the incident was just "the beginning" but stopped short of saying Ukrainian forces were responsible for the blast.
"Everything illegal must be destroyed, everything that is stolen must be returned to Ukraine, everything occupied by Russia must be expelled," Mykhailo Podolyak wrote.
Putin has instructed the government to create a state commission to investigate the incident, TASS news agency reported, citing Kremlin spokesman Dmitry Peskov.
Interfax quoted the chairman of the Crimean republic's State Council, Vladimir Konstantinov, as saying that "Ukrainian vandals managed to reach for the Crimean bridge with their bloody hands".
"They now have something to be proud of," he said. "In their 23 years of economic activity they failed to build anything worthy of attention in Crimea, but (now) they have managed to damage the roadway of the Russian bridge."
Konstantinov said the damage was "not of a severe nature" and would be quickly repaired.
The head of Ukraine's National Security and Defence Council, Oleksiy Danilov posted a video of the burning bridge on social media alongside a video of Marilyn Monroe singing "Happy birthday, Mr President".
Putin turned 70 on Friday.