Russian President Vladimir Putin has called the attack on the Kerch Bridge to Crimea "a terrorist act" carried out by Ukrainian special services.
A criminal terror investigation into the explosion that damaged a prominent Russian landmark has been launched by the country's investigative committee.
What Russian authorities are calling a truck bomb on Saturday hit the huge bridge linking Russia with the Crimean Peninsula, which Moscow annexed eight years ago from Ukraine.
Putin is to chair a meeting of his security council today – to discuss the explosion which damaged a key bridge linking Russia with occupied Crimea.
Road and rail traffic on the bridge was temporarily halted, damaging an important supply route for the Kremlin's forces and dealing a sharp blow to Russian prestige.
"There's no doubt it was a terrorist act directed at the destruction of critically important civilian infrastructure of the Russian Federation," Mr Putin said in a video of a meeting on Sunday with the chairman of the investigative committee, Alexander Bastrykin.
"And the authors, perpetrators, and those who ordered it are the special services of Ukraine."
Mr Bastrykin said Ukrainian special services and citizens of Russia and other countries took part in the attack.
"We have already established the route of the truck," he said, adding it had been to Bulgaria, Georgia, Armenia, North Ossetia and Krasnodar – a region in southern Russia – among other places.
Watch: Truck bomb leads to partial collapse of bridge linking Russia and Crimea.
The statements followed overnight Russian missile strikes on the city of Zaporizhzhia that brought down part of a large apartment building, leaving at least a dozen people dead.
The six missiles used in Sunday's overnight attack were launched from Russian-occupied areas of the Zaporizhzhia region, the Ukrainian air force said.
The region is one of four Russia claimed as its own this month, though its capital of the same name remains under Ukrainian control.
Russia has suffered a series of setbacks nearly eight months after invading Ukraine in a campaign many thought would be short-lived.
In recent weeks, Ukrainian forces have staged a counter-offensive, retaking areas in the south and east, while Moscow's decision to call up more troops has led to protests and an exodus of tens of thousands of Russians.
Recent fighting has focused on the regions just north of Crimea, including Zaporizhzhia.
Ukrainian President Volodymyr Zelenskyy lamented the latest attack in a Telegram post.
"Again, Zaporizhzhia. Again, merciless attacks on civilians, targeting residential buildings, in the middle of the night," he wrote.
At least 19 people died in Russian missile strikes on apartment buildings in the city on Thursday.
"From the one who gave this order, to everyone who carried out this order: They will answer," he added.
Ukrainian Foreign Minister Dmytro Kuleba called the attacks on civilians a war crime and urged an international investigation.
Mr Putin personally opened the Kerch Bridge in May 2018 by driving a truck across it as a symbol of Moscow's claims on Crimea.
The bridge, the longest in Europe, is vital to sustaining Russia's military operations in southern Ukraine. No one has claimed responsibility for damaging it.