Russia on Saturday opened a criminal probe into a blast that blew up a truck and heavily damaged a vital bridge linking the Crimea peninsula to Russia.

The blast on the strategic road-and-rail bridge earlier in the day brought down sections of road, prompting gleeful messages from Ukrainian officials.

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.

Russia's investigative committee said it had "initiated a criminal case in connection with the incident on the Crimean bridge," adding that "a truck was blown up."

"According to preliminary information, this morning on the automobile part of the Crimean bridge from the side of the Taman Peninsula, a truck was blown up, which caused seven fuel tanks to ignite on a train heading towards the Crimea peninsula," the committee said on Saturday. "As a result, two lanes partially collapsed."

Dramatic videos on social media show the giant bridge on fire and partially collapsed into the sea. Later, the Emergency Ministry said the fire had been extinguished, the Russian news agency Interfax reported.

The investigative committee said it had sent detectives to the scene.

"Investigators of the Russian Investigative Committee are establishing all the circumstances of the incident and persons involved in the crime."

Hours later, the committee said three people so far have been found dead as the result of the explosion on the bridge.

"They are believed to be passengers of a car that was near the truck that exploded. The bodies of two victims, a man and a woman, have already been recovered from the water and their identities are being established," the committee said in a statement.

The investigators have also established the details of the truck and its owner, registered in Russia's southern Krasnodar region, and begun searching his place of residence, it added.

The incident on the Crimean bridge was a special operation carried out by the Security Service of Ukraine (SSU), the Interfax-Ukraine News Agency, a Kyiv-based Ukrainian news agency, cited its source in Ukraine's law enforcement agencies as saying. The SSU hasn't commented on the incident yet.

Since the start of the Russia-Ukraine conflict on February 24, Ukrainian officials 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 Defense 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."

Russia took over Crimea from Ukraine in 2014. And it now represents a crucial supply route for the Russian forces who have taken control of most of southern Ukraine's Kherson region.

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Zaporizhzhia shelling

Also on Saturday, Russia and Ukraine traded blame for an overnight shelling that cut the power line supplying cooling systems at the Russian-controlled Zaporizhzhia nuclear power plant in south-eastern Ukraine.

Ukraine's state nuclear company Energoatom said the plant was now getting power to cover its own needs from its backup diesel generators.  

"The diesel generators started automatically. The available supplies of diesel fuel for their operation in this mode will be enough for 10 days," the company wrote on Telegram. 

However, according to Russia Today, the plant automatically switched to diesel power generators because of the attack from the Ukrainian side.

The International Atomic Energy Agency (IAEA), the UN atomic watchdog, said the shelling on the facility was "tremendously irresponsible," calling again for a protection zone.

"The resumption of shelling, hitting the plant's sole source of external power, is tremendously irresponsible," the IAEA quoted its chief Rafael Grossi as saying, confirming that the plant is now relying on diesel generators.

Grossi will visit Russia and Ukraine "soon" to discuss setting up a protection zone at the plant, it added.

The safety of the Zaporizhzhia plant, one of Europe's largest nuclear power plants, has been a major concern since the beginning of the Russia-Ukraine conflict in late February; the former says it is a "special military operation" while the latter calls it a "land grab."

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(With input from agencies)