This video grab shows thick black smoke rising from a fire on the Kerch bridge that links Crimea to Russia. AFP

Russia said the blast set ablaze seven oil tankers by transported by train and collapsed two car lanes of the giant road and rail structure.

Dramatic social media footage showed the bridge on fire with parts plunging into the sea.

"Today at 6:07 am (0307 GMT) on the road traffic side of the Crimean bridge ... a car bomb exploded, setting fire to seven oil tankers being carried by rail to Crimea," Russian news agencies cited the national anti-terrorism committee as saying.

The bridge, personally inaugurated by President Vladimir Putin in 2018, is a vital transport link for carrying military equipment to Russian soldiers fighting in Ukraine.

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Moscow had maintained the bridge crossing was safe despite the fighting.

The Kremlin spokesman said Putin had ordered a commission to be set up to look into the blast, Russian news agencies reported.

Russia's powerful investigative committee opened a criminal probe into the explosion and sent detectives to the scene.

A helicopter drops water to extinguish fuel tanks ablaze in the Kerch Strait, Crimea, on Saturday. Reuters

It said a truck exploded "on the automobile part of the Crimean bridge from the side of the Taman Peninsula".

This "caused seven fuel tanks to ignite on a train heading towards the Crimea Peninsula. As a result, two lanes partially collapsed."

While officials in Moscow stopped short of blaming Kyiv, an official in Russian-installed Crimea pointed the finger at "Ukrainian vandals".

Mykhailo Podolyak, adviser to the head of the office of Ukraine's presidency, took to Twitter posting a picture of a long section of the bridge half-submerged in the waters.

Agence France-Presse