A powerful blast has damaged the Kerch Bridge, the sole road and rail link between Russia and the Crimean peninsula.
Three people died when the bridge caught fire on Saturday, causing it to partially collapse.
Russian President Vladimir Putin accused Ukraine of orchestrating the blast, even likening the explosion to an "act of terrorism", but Ukraine officials take no claim of responsibility.
So, what's really going on? Why is this bridge so important? And what's the fallout from the explosion?
What is the history of the Kerch bridge?
Russia seized Crimea from Ukraine in 2014.
Four years later, Mr Putin opened the 19-kilometre bridge stretching along the Kerch Strait between the Taman Peninsula of Krasnodar Krai in Russia and the Kerch Peninsula of Crimea in Ukraine.
It was a flagship project for Mr Putin, who opened it himself for road traffic with great fanfare by driving a truck across it.
The longest bridge in Europe, it is the only direct link between the transport network of Russia and the Crimean peninsula.
Why is the bridge so important?
Not only a prestigious symbol of Moscow's annexation, the $US3.6 billion ($AU5.6 billion) bridge is also a key supply route to forces battling to hold territory captured in southern Ukraine.
It holds both important strategic and symbolic value to Russia in its faltering war in Ukraine.
As well, it is crucial for the supply of fuel, food and other products to Crimea, where the port of Sevastopol is the historic home base of Russia's Black Sea Fleet.
It became a major supply route for Russian forces after Moscow invaded Ukraine, sending forces from Crimea to seize most of southern Ukraine's Kherson region and some of the adjoining Zaporizhzhia province.
The damage to the bridge came amid battlefield defeats for Russia and growing concerns that Moscow could resort to nuclear weapons, after Mr Putin repeatedly cautioned the West that any attack on Russia could provoke a nuclear response.
What was the cause of the blast?
Russian authorities have been quick to report the explosion was caused by a truck bomb, which they say was carried out by Ukrainian secret services.
However, nothing has been confirmed.
Russia's National Anti-Terrorism Committee said a truck bomb caused seven railway cars carrying fuel to catch fire, resulting in the "partial collapse of two sections of the bridge".
In a video on the Kremlin's Telegram channel, the head of Russia's Investigative Committee also presented findings of an inquiry into the cause, saying the vehicle had travelled through Bulgaria, Georgia, Armenia, North Ossetia and Russia's Krasnodar region before reaching the bridge.
He said Ukrainian special services and citizens of Russia and other countries took part in the act.
How did Ukraine react?
In his evening address, Ukraine's president, Volodymyr Zelenskyy, poked fun at the explosion, but did not claim responsibility.
"Today was not a bad day and mostly sunny on our state's territory," he said. "Unfortunately, it was cloudy in Crimea. Although it was also warm."
Ukraine's national security and defence council secretary Oleksiy Danilov tweeted a video of the fire and of Marilyn Monroe singing Happy Birthday Mr President following Mr Putin's 70th birthday on Friday.
Postage stamps, artwork and commemorative credit cards are already being printed.
Ukrposhta — the Ukrainian postal service — unveiled a new stamp on Twitter on Monday featuring the damaged Kerch bridge. The stamp also featured two figures bearing a striking resemblance to Leonardo DiCaprio and Kate Winslet's famous Titanic pose.
In Kyiv, Ukrainians posed with an oversize postage stamp depicting the bridge explosion.
What was Russia's response?
Mr Putin accused Ukraine of orchestrating what he called a terrorist attack on the bridge.
"There is no doubt. This is an act of terrorism aimed at destroying critically important civilian infrastructure," Mr 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."
Security Council deputy chairman Dmitry Medvedev said that Russia should kill the "terrorists" responsible for the attack.
ABC/AP/Reuters
