Russian President Vladimir Putin accused Ukraine of orchestrating the explosion of a key bridge linking Russia and Crimea, an act he described as terrorism.
"There is no doubt. This is an act of terrorism aimed at destroying critically important civilian infrastructure," Putin said on Sunday in a video on the Kremlin's Telegram channel.
"This was devised, carried out and ordered by the Ukrainian special services," said Putin. He will hold a meeting on Monday of his security council, Kremlin spokesperson Dmitry Peskov was quoted as saying by Russian state news agency Tass.
The blast on Saturday on the bridge over the Kerch Strait, a key supply route for Moscow's forces in southern Ukraine, had prompted gleeful messages from Ukrainian officials but no claim of responsibility.
The bridge is also a major artery for the port of Sevastopol, where the Russian Black Sea fleet is based.
The damage to the bridge, which had been an imposing symbol of Russia's annexation of the Crimean peninsula, came amid battlefield defeats for Russia.
It also took place at a time when concerns mounted that Russia could resort to using nuclear weapons after Putin in recent weeks repeatedly cautioned the West that any attack on Russia could provoke a nuclear response.
On Sunday, Putin met Alexander Bastrykin, the head of Russia's Investigative Committee, who presented findings of an inquiry into what he said was Saturday's explosion of a vehicle and subsequent fire on the bridge.
Speaking on camera, Bastrykin said investigators had established the route that the vehicle had taken and the individuals who were involved in its movements.
He said that it had gone through Bulgaria, Georgia, Armenia, North Ossetia and Russia's Krasnodar region before arriving on the bridge. Among those who helped Ukrainian special services prepare were "citizens of Russia and foreign countries," Bastrykin added.
A criminal case has been launched on the explosion, Andrei Kartapolov, chairman of the defence commmittee of the State Duma, Russia's lower house of parliament, told the Vedomosti news site, adding that "guilty parties" would be named.
Asked whether Moscow would respond if it is found that Kyiv was behind the incident, he said: "There will be a response. We will have to see just what that response will be."
Images showed part of the bridge's roadway blown away, although rail services and partial road traffic resumed.
The Russian transport ministry, quoted by RIA news agency, said nearly 1,500 people and 162 heavy cargoes had traveled by ferry across the Kerch Strait since the explosion.
Russia seized Crimea from Ukraine in 2014 and the 19-km (12-mile) bridge linking the region to its transport network was opened with great fanfare four years later by Putin.
Russia's defence ministry said on Saturday its forces in southern Ukraine could be "fully supplied" through existing land and sea routes.
Hours before Putin's statement, Russian missiles struck an apartment block and other residential buildings in Ukraine's southeastern city of Zaporizhzhia, killing at least 14 people and wounding more than 70.
The pre-dawn strikes were the second such attack against the city in three days.
Russian aircraft launched at least 12 missiles, partially destroying a nine-storey apartment block, levelling five other residential buildings and damaging many more, Oleksandr Starukh, governor of the region, said on state-run television.
The wounded included 11 children, Ukrainian President Volodymyr Zelenskiy said, condemning the attack as "absolute evil", vowing those responsible would be brought to justice.
"This was a deliberate hit. Whoever gave the order and whoever carried it out knew what they were targeting," he said in a video address.
Zaporizhzhia city, about 52 km (30 miles) from a Russian-held nuclear power plant, has been under frequent shelling in recent weeks, with 19 people killed on Thursday. Emergency workers and firefighters cordoned off the nine-storey building and dug for survivors and casualties in the smouldering rubble of a massive central section that had collapsed.
The blast wrecked cars and left torn metal window frames, balconies and air conditioners dangling from the building's shrapnel-pitted facade.
Most of the Zaporizhzhia region, including the nuclear plant, have been under Russian control since the early days of Russia's invasion in February. The capital of the region, Zaporizhzhia city, remains under Ukrainian control.