A huge blast on Russia's bridge to Crimea, the Ukrainian peninsula it annexed in 2014, has killed three people, investigators say.
The victims were in a nearby car when a lorry blew up, the Russian investigators said, bringing down sections of the bridge's roadway.
Russia says the railway part of the bridge - where oil tankers caught fire - will reopen on Saturday.
They also claim road traffic will begin on an undamaged lane this evening, according to BBC.
The rail and road crossing opened in 2018 and was a symbol of Russia's illegal annexation.
An adviser to Ukraine's President Volodymyr Zelensky, Mykhailo Podolyak, did not directly claim Ukrainian responsibility but wrote: "Crimea, the bridge, the beginning.
"Everything illegal must be destroyed, everything stolen must be returned to Ukraine, everything occupied by Russia must be expelled."
Ukraine's defence ministry compared the bridge explosion to the sinking of Russia's Moskva missile cruiser in April.
"Two notorious symbols of Russian power in Ukrainian Crimea have gone down," it tweeted. "What's next in line?"
Meanwhile the Ukrainian government simply tweeted: "Sick burn."
The Russian foreign ministry said: "The Kiev [Kyiv] regime's reaction towards destruction of civilian infrastructure is a testament to its terrorist nature."
It is hard to exaggerate the significance, and symbolism, of seeing the bridge - which was opened by President Putin - on fire.
Russia has used the bridge to move military equipment, ammunition, and personnel from Russia to battlefields in southern Ukraine.
As such, Ukrainian authorities said it was a legitimate target, as they vow to retake the peninsula.
Any attack on Crimea, where the Russian army has a massive presence, will be seen as another massive humiliation for the Kremlin.
The bridge is particularly hated by Ukrainians. Social media in Ukraine erupted in celebration on seeing the fire - one day after Russian President Vladimir Putin turned 70.
Local authorities in Crimea say they will organise a ferry service between the Russian mainland and the peninsula.
Russia's National Anti-Terrorism Committee said: "At 06:07 Moscow time today [03:07 GMT], an explosion was set off at a cargo vehicle on the motorway part of the Crimean bridge on the side of the Taman peninsula, which set fire to seven fuel tanks of a train that was en route to the Crimean peninsula.
"Two motorway sections of the bridge partially collapsed."
Crimean parliamentary speaker Vladimir Konstantinov blamed the explosion on "Ukrainian vandals, who have finally managed to reach their bloody hands to the Crimean bridge".
He added the damage to the bridge would be "promptly restored, since it is not of a serious nature".
Russian President Vladimir Putin has been briefed about the "emergency" on the bridge and has ordered a government inquiry, Kremlin spokesman Dmitry Peskov said. A criminal investigation is also under way.
The 19km (12-mile) bridge across the Kerch Strait, which cost £2.7bn to build, was opened four years after Moscow illegally annexed Crimea.
It is the longest bridge in Europe, and was hailed by Russian media as "the construction of the century". Russian officials previously claimed it was well protected from threats from air, land or water.
The crossing is more than 100 miles from Ukrainian-held territory. One explosives expert told the BBC the fire was probably not caused by a missile.
"The lack of obvious blast / fragmentation damage on the road surface suggests that an air-delivered weapon was not used," he said.
He said it was possible that "a well-planned attack from below may have been the cause".
"I suspect explosives on the road bridge and train deck were initiated near simultaneously using coded radio command," he added.
Ukraine claimed responsibility last month for a series of air strikes on Crimea over the summer, including an attack on Russia's Saky military base.
Kyiv has the momentum in this conflict. The army has reclaimed large swathes of territory, forcing Russian troops to abandon long-held positions.
Amid the losses, Moscow has begun a chaotic military mobilisation - which led to rare anti-war protests in Russia, and a huge exodus of military-age men.
On Russian TV talk shows, presenters and studio guests have been expressing increasing doom and gloom about the situation.