The blast on the Kerch bridge, 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 19km bridge linking it to Russia's transport network was opened with great fanfare four years later by President Vladimir Putin himself.

It now represents a major supply route 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 Saturday's blast was a deliberate attack or an accident but the damage to such high-profile infrastructure came at a time when Russia has suffered several battlefield defeats.

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.

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

The Russian National Anti-Terrorism Committee said a freight truck had blown up on the bridge's roadway at 6.07am local time, causing seven fuel tanker wagons to catch fire on a train heading for the peninsula on the bridge's upper level.

It said two spans of road bridge had partially collapsed, but 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 the road bridge was still intact in one direction, although traffic was suspended while the damage was assessed.

Images posted on social media showed one half of the roadway blown away, and the other half still attached, but cracked, while a train on the upper bridge was engulfed in flames. The images could not immediately be verified.

Others taken from a distance showed thick smoke pouring from part of the bridge.

An adviser to Ukrainian President Volodymyr Zelenskiy posted a message on Twitter saying the incident was just "the beginning" but stopped short of saying Ukrainian forces were responsible for the blast.

On Saturday, hundreds of people who had hoped to drive to Crimea from the Russian town of Kerch were redirected to the ferry port, only to find that high winds were preventing any sailings.

In a video message Aksyonov, the Crimea governor, said he wanted to "assure Crimeans that the Republic of Crimea is fully provided with fuel and food. We have more than a month's worth of fuel, and more than two months' worth of food".

However, the Russian Energy Ministry said on Telegram that Crimea had only 15 days of motor fuel.

Australian Associated Press