A predawn assault on a critical bridge linking the occupied Crimean Peninsula to mainland Russia forced the temporary closure on Monday of a main artery used by its military to support its troops in southern Ukraine, in yet another blow to a Russian military command that was already dealing with internal strife.

Hours after the attack, Moscow announced that it was pulling out of the Black Sea grain deal, an agreement that helped keep global food prices stable. But Dmitri S. Peskov, the Kremlin’s spokesman, said the attack on the Kerch Strait Bridge and Russia’s decision to suspend its participation in the deal were not connected.

Given the deep strategic and symbolic importance of the bridge, Monday’s assault was another embarrassment for Russia’s military leadership, which has been roiled by the fallout from last month’s failed mutiny by the Wagner mercenary group.

Russian officials said two people were killed in the attack and a third was injured. The extent of the damage remained unclear, but the assault again highlighted the vulnerability of this key piece of infrastructure far from the front lines.

Rail service over the bridge resumed Monday morning. But damage to the car lanes — which appeared to leave part of the road tilting, according to video verified by The New York Times — threatened to constrict Russian logistical operations. The top Russian-installed official in Crimea, Sergei Aksyonov, said on the Telegram messaging app that drivers should seek alternative routes.

If the bridge were destroyed or severely damaged, Moscow would be left with a single major land route from Russia along the southern coast of Ukraine to support tens of thousands of soldiers fighting to hold onto territory captured in the first weeks of the invasion.

Pro-war Russian military bloggers and commentators were quick to use the attack on the bridge as evidence of what they said was another failure by the Russian military command. Igor Girkin, a former Russian intelligence officer who runs a prominent blog about military affairs, said that Ukraine would strike again and again until the link is severed.

The attack came as Ukrainian forces were engaged in a grinding five-week-long counteroffensive aimed at driving Russian forces from areas of southern and eastern Ukraine. Russian forces are dug in behind fields laden with land mines, so the Ukrainian military has been forced to move cautiously and progress has been slow.

Isolating Russian forces in Crimea is an essential part of the Ukrainian counteroffensive strategy, according to analysts. Ukrainian ground forces have been seeking to drive a wedge through the natural land bridge that connects Russia to the peninsula through southern Ukraine, and have repeatedly targeted the bridge that President Vladimir V. Putin ordered be built after Russia illegally annexed Crimea in 2014.

Ukrainian officials offered no comment on the incident on Monday. But they have previously said that the 12-mile-long structure, a road and railway bridge that run in parallel, is a legitimate target because of its vital logistical role in the Kremlin’s war effort.

On Monday, Ukrainian officials celebrated the attack even as they maintained a studied policy of strategic ambiguity, declining to comment on any possible Ukrainian role.

“Any illegal structures used to deliver Russian instruments of mass murder are necessarily short-lived, regardless of the reasons for the destruction,” Mykhailo Podolyak, a senior adviser to President Volodymyr Zelensky of Ukraine, said in a statement.

Monday’s assault came a little more than nine months after an Oct. 8 attack on the bridge by an explosives-laden truck forced the closure of one lane of traffic and damaged the railroad tracks.

Ukraine did not officially claim responsibility for that attack until May of this year, when the head of Ukraine’s security service, Vasyl Maliuk, acknowledged that the intelligence services took “certain measures” that allowed for the assault.

The National Anti-Terrorism Committee of the Russian Federation said in a statement that Ukraine attacked the bridge Monday using two maritime drones, a claim that could not be independently verified. Video and photographs verified by The Times showed damage to both sides of the road bridge, with the most significant being along a span of the bridge heading into Russia. One photo also showed a damaged car on the bridge.

Milana Mazaeva and Ivan Nechepurenko contributed reporting.

— Marc Santora, Neil MacFarquhar and Haley Willis