KYIV, Ukraine — A fireball consumed two sections of the only bridge linking the occupied Crimean Peninsula to Russia on Saturday, disrupting the most important supply line for Russian troops fighting in southern Ukraine and dealing an embarrassing blow to the Kremlin, which is facing continued losses on the battlefield and mounting criticism at home.
The blast and fire sent part of the 12-mile Kerch Strait bridge tumbling into the sea and killed at least three people, according to Russian authorities, who said a Ukrainian truck bomb had caused the blast.
The Ukrainian government, which applauded the damage, did not publicly take responsibility for the explosion. One senior Ukrainian official, speaking on condition of anonymity because of a ban on officials discussing the matter, said that Ukraine’s intelligence services had orchestrated the attack and that it involved a bomb loaded onto a truck that drove across the bridge.
It was unclear if the driver of the truck, who appeared to have died in the blast, was aware there were explosives inside. In video captured by a surveillance camera on the bridge, a huge fireball is seen. A small sedan and a tractor-trailer truck driving side by side appear at the epicenter of the blast.
For President Vladimir V. Putin of Russia, who presided over the bridge’s opening in 2018, the explosion was a highly personal affront, underscoring his difficulties in the face of continued Ukrainian successes.
The full extent of the damage was not immediately clear. The bridge has spans for train and automobile traffic.
By Saturday evening, the railroad section of the bridge had undergone repairs and a train with 15 cars had successfully crossed the span, according to a Russian state news agency, Tass. Car traffic had also resumed on the undamaged side of the bridge, the head of Crimea, Sergei Aksyonov, said in a post on Telegram.
Even so, Russian officials and hard-line military bloggers were already calling for revenge, with one member of Crimea’s Parliament warning that anything less than an “extremely harsh” response would show weakness.
The explosion is emblematic of a Russian military in disarray. Russian forces were unable to protect the bridge, despite its centrality to the Russian war effort, its personal importance to Mr. Putin and its potent symbolism as the literal connection between Russia and Crimea.
Any serious impediment to traffic on the bridge could have a profound effect on Russia’s ability to wage war in southern Ukraine, where Ukrainian forces have been fighting an increasingly effective counteroffensive.
The bridge is the primary military supply route linking Russia with the Crimean Peninsula. Without it, analysts said, the Russian military will be severely limited in its ability to bring fuel, equipment and ammunition to Russian units fighting in the Kherson and Zaporizhzhia regions, two of the four Ukrainian provinces that Mr. Putin announced Russia had annexed on Sept. 30.
Russia’s National Anti-Terrorism Committee said in a statement that a truck had exploded on the automobile side of the bridge, igniting seven fuel cisterns being pulled by a train on a parallel crossing headed in the direction of Crimea. The explosion caused two sections of the bridge to partly collapse.
While there was no official claim of responsibility, Russian and Ukrainian officials indicated that the explosion was no accident, and top Ukrainian officials, who in the past have said the bridge would be a legitimate target for a Ukrainian strike, made no secret of their satisfaction.
“Crimea, the bridge, the beginning,” Mykhailo Podolyak, an adviser to Ukraine’s president, wrote in a Twitter post on Saturday. “Everything illegal, must be destroyed. Everything stolen returned to Ukraine. All Russian occupiers expelled.”
The president, Volodymyr Zelensky, seemed to allude to the attack when he noted in his nightly address that Saturday “was a good and mostly sunny day” in Ukrainian territory. “Unfortunately, it was cloudy in Crimea,” he said.
Ukraine’s domestic intelligence agency, the Security Service of Ukraine, known by its Ukrainian acronym S.B.U., issued a statement rephrasing a stanza of a poem by Ukraine’s national poet, Taras Shevchenko. “Dawn, the bridge is burning beautifully,” the agency posted on Twitter. “A nightingale in Crimea meets the S.B.U.”
Maria Varenikova contributed reporting from Kryvyi Rih, Ukraine, and Megan Specia from Kyiv, Ukraine.