KHARKIV, Ukraine —  

Russian authorities said that a truck bomb on Saturday caused a fire and the collapse of a section of a bridge linking Russian-occupied Crimea with Russia — a key supply artery for Moscow’s faltering war effort in southern Ukraine.

The attack on the bridge comes a day after Russian President Vladimir Putin turned 70, dealing his forces the latest blow that observers fear may lead him to up the ante in his war on Ukraine.

Russia’s National Anti-Terrorism Committee said that the truck bomb caused seven railway cars carrying fuel to catch fire, resulting in a “partial collapse of two sections of the bridge.” The committee didn’t immediately apportion blame.

The Crimean peninsula holds symbolic value for Russia and is key to sustaining its military operations in the south. If the bridge is made inoperable, it would make it significantly more challenging to ferry supplies to the peninsula. Russia seized the areas north of Crimea early in the war and built a land corridor to it along the Sea of Azov, but more recently Ukraine is pressing a counteroffensive to reclaim them.

The bridge has train and automobile sections. The Russian National Anti-Terrorism Committee specified that the explosion and fire led to the collapse of the two sections of one of the two links of the automobile bridge, while another link was intact.

Authorities have suspended commuter train traffic across the bridge until further notice. Putin was informed about the explosion, and he ordered the creation of a government panel to deal with the emergency

The 12-mile bridge across the Kerch Strait linking the Black Sea and the Sea of Azov is the longest in Europe. It has provided an essential link to the Crimean peninsula, which Russia illegally annexed from Ukraine in 2014. Ukrainian officials have repeatedly threatened to strike the bridge.

Russia opened the first part of the span to car traffic in May 2018. The parallel bridge for rail traffic opened the following year.

The $3.6-billion project is a tangible symbol of Moscow’s claims on Crimea. It was Russia’s only land link to the peninsula until Russian forces seized more Ukrainian territory on the northern end of the Sea of Azov in heavy fighting, particularly around the city of Mariupol, earlier this year.

In August, Russia suffered a series of explosions at an air base and munitions depot in Crimea, which underlined its vulnerability.

The truck bomb on the bridge occurred hours after explosions rocked the eastern Ukrainian city of Kharkiv early Saturday, sending towering plumes of smoke into the sky and triggering a series of secondary explosions.