Ukraine said on Tuesday that it had struck the Crimean bridge for the third time after spending months planting explosives on support structures beneath the water.

The extent of the damage was not immediately clear, but the Security Service of Ukraine, the intelligence agency known as the S.B.U, released a statement and a video that it said showed one of the detonations.

“Today, at 4:44 a.m., without any civilian casualties, the first explosive device was detonated,” the agency said in a statement. “The underwater support pillars were severely damaged at the seabed level — aided by the equivalent of 1,100 kg of TNT. As a result, the bridge is effectively in an emergency condition.”

The claim could not immediately be independently confirmed.

It came two days after Ukraine conducted one of the most ambitious operations of the war deep inside Russia, targeting the Russian strategic bombing fleet.

RIA Novosti, a Russian state news agency, reported that the bridge was briefly closed for several hours on Tuesday. An app that is widely used in Russia to track traffic on the bridge showed that it was open again as of midafternoon.

The 12-mile Kerch Strait Bridge links the Crimean Peninsula to Russia and is a primary supply route for Moscow’s forces fighting in southern Ukraine. It is also deeply symbolic for President Vladimir V. Putin of Russia, who presided over the bridge’s opening in 2018.

In October 2022, a truck laden with explosives was detonated as it crossed the bridge, causing a fireball large enough to rupture fuel tanks on a passing train and setting it on fire. The blast pulled part of the roadway off its joints and into the sea.

The Russians set about repairs but 10 months later, the bridge was hit again, this using maritime drones to target support pillars.

After each attack, the Russians worked to repair the vital artery and increased the defenses around it.

Ivan Nechepurenko contributed reporting.