The whole operation took several months, it added.

The agency said it had used 1100 kilograms of explosives which “severely damaged” the underwater pillars supporting the bridge.

According to Ukraine’s security service, the SBU mentioned on Telegram that their agents had strategically placed explosives on the bridge’s road and rail piers, also known as the Kerch Bridge, and set off the first explosion at 4:44 a.m. local time on Tuesday (11:44 a.m. AEST).

Initially, the bridge traffic was halted early on Tuesday morning, then paused again in the mid-afternoon before ultimately resuming just before 6:00 p.m. local time (1:00 a.m. Wednesday AEST). While the full extent of the damage was not immediately assessed, Tuesday’s operation underscores the SBU’s efforts to catch Moscow off guard and signal the ongoing costs of its military campaign.

On Sunday, the SBU launched an audacious drone attack on Moscow’s fleet of nuclear-capable bombers, stationed at various Russian airfields thousands of miles away from Ukraine.

Vasul Malyuk, the head of the SBU, said that attack caused an estimated $US7 billion ($10.8 billion) in damage and had struck 34 per cent of Russia’s strategic cruise missile carriers, which have been used to pummel Ukrainian cities throughout the war.

The SBU said Malyuk had also overseen Tuesday’s attack.

“God loves the Trinity, and the SBU always sees things through to the end and never does the same thing twice. We previously struck the Crimean Bridge twice, in 2022 and 2023. So today we continued this tradition, this time underwater,” Malyuk said.

He stressed that the bridge is a “completely legitimate target,” since Russia uses it “as a logistical artery to supply its troops” fighting in mainland Ukraine.

As well as serving as a vital supply line for Moscow’s troops, the Crimean Bridge also has huge symbolic value for President Vladimir Putin, embodying his objective to bind the Ukrainian peninsula to Russia.

Built after Russia’s annexation of Crimea in 2014, Putin opened the bridge in 2018. The project cost around $US3.7 billion ($5.7 billion).

Tuesday’s attack marks the third time that Ukraine has targeted the bridge since Moscow’s full-scale invasion in 2022.

In October of that year, a fuel truck exploded on the bridge, engulfing a part of it in flames.

In July 2023, the SBU said it had blown up a part of the bridge using an experimental sea drone. Both times, Russia moved quickly to repair the damaged sections.

As well as suspending traffic on the bridge, Russian authorities temporarily halted maritime traffic in the waters off Sevastopol, the largest city in Crimea, according to state media RIA Novosti.