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

"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.

It is hugely important to the Kremlin and Moscow had maintained the bridge crossing was safe despite the fighting.

Ukraine's presidential advisor Mykhailo Podolyak earlier took to Twitter posting a picture of a long section of the bridge half-submerged.

"Crimea, the bridge, the beginning," he wrote.

"Everything illegal must be destroyed, everything stolen must be returned to Ukraine, everything occupied by Russia must be expelled."

The Ukrainian post office announced it was preparing to print stamps showing the "Crimean bridge -- or more precisely, what remains of it".

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. 

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." 

Officials in Moscow stopped short of blaming Kyiv.

But an official in Russian-installed Crimea pointed the finger at "Ukrainian vandals." Another in the neighbouring Kherson region said repairs could "take two months". 

And the spokeswoman of Russia's foreign ministry said that Kyiv's reaction to the blasts showed its "terrorist nature." 

'Undisguised terrorist war'

"There is an undisguised terrorist war against us," Russian ruling party deputy Oleg Morozov told the RIA Novosti news agency. 

"If we stay quiet and do not give an adequate response, then such attacks will multiply," he said.

There have been several explosions at Russian military installations in the Crimean peninsula.

If it is established that Ukraine was behind the latest blast, alarm bells may sound with the bridge so far from the front line.

The blasts come after Ukraine's recent lightning territorial gains in the east and south that have undermined the Kremlin's claim that it annexed Donetsk, neighbouring Lugansk and the southern regions of Zaporizhzhia and Kherson. 

The Moscow-installed head of the peninsula, Sergei Aksyonov, called on Crimeans to remain "calm" as authorities appeared to downplay the blasts. 

"I call on everyone to clam down and not spread fake information," he said on Telegram. "The situation is being controlled, professionals are working on the ground."

He said rail links to Russia had been halted and added that authorities had set up food and heating points to help stranded drivers. 

Authorities also tried to calm fears of food and fuel shortages in Crimea, which is fully reliant on the Russian mainland since Moscow annexed it in 2014. 

Russia's transport ministry said a ferry service has been launched. Its energy ministry told agencies that the peninsula is fully provided with fuel.

The blasts came a day after President Vladimir Putin's 70th birthday. 

Some Russian gains

Russian forces said Friday they had captured ground in Donetsk in east Ukraine, their first claim of new gains since a Kyiv counter-offensive rattled Moscow's war effort.

Separatist forces in the war-battered Donetsk region said they had retaken a series of villages near the Ukraine-controlled industrial town of Bakhmut, which has been under Russian shelling for weeks.

The Donetsk region, which has been partially controlled by Kremlin-backed separatists for years, is a key prize for Russian forces, which sent troops to Ukraine in February.

But Kyiv's forces have in recent weeks been pushing back against Russian soldiers across the front lines in the south and in the east, including in parts of Donetsk.

Ukrainian President Volodymyr Zelensky said late Friday his forces had recaptured nearly 2,500 square kilometres (965 square miles) in the counter-offensive that began late last month. 

Zelensky has pushed to punish Russia in other areas, urging Brussels to ramp up pressure on its energy sector -- a day after the EU imposed a fresh round of sanctions on Moscow.

In the more than seven months since Russia's offensive, Putin has made thinly veiled threats of using nuclear weapons. 

US President Joe Biden on Thursday warned the world was facing "Armageddon" as Putin may use his atomic arsenal.

But by Friday the White House dialled back the alarm, saying the president's comments did not reflect new intelligence.