A powerful truck explosion seriously damaged Russia’s road-and-rail bridge to Crimea on Saturday, hitting a prestige symbol of Moscow’s annexation of the peninsula and the key supply route to Russian forces battling to hold territory captured in southern Ukraine.

The blast on the bridge over the Kerch Strait, for which Russia did not immediately assign blame, prompted gleeful messages from Ukrainian officials but no direct claim of responsibility.

Russian investigators said three people had been killed, probably the occupants of a car travelling near the truck that blew up.

Russia seized Crimea from Ukraine in 2014 and the 12-mile Kerch bridge linking it to Russia’s transport network was opened with great fanfare four years later by President Vladimir Putin, who drove a construction truck across it.

It now represents a major artery for the Russian forces who have taken control of most of southern Ukraine's Kherson region, and for the naval port of Sevastopol, whose governor told locals: "Keep calm. Don't panic."

It was not yet clear if the blast was a deliberate attack, but the damage to such high-profile infrastructure came at a time when Russia has suffered several battlefield defeats and could further cloud the Kremlin's messages of reassurance to its public that the conflict is going to plan.

It also took place a day after Putin's 70th birthday.

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".

Since the beginning of Russia's military operations in Ukraine February, officials in Ukraine have made regular allusions to their desire to destroy the Kerch bridge. Ukraine's postal service said on Saturday it would print a special stamp to commemorate the blast.

Russia's Defence Ministry said in a statement that its forces in southern Ukraine could be "fully supplied" through existing land and sea routes, and the Transport Ministry said rail traffic across the bridge would resume by 1700 GMT.

Foreign Ministry spokeswoman Maria Zakharova said Kyiv's reaction to the destruction of civilian infrastructure "testifies to its terrorist nature". (Reuters)