Ukraine’s domestic security service, the SBU, said Monday that it struck a Russian submarine in the Black Sea port of Novorossiysk, causing critical damage to the vessel and effectively disabling it.

In a statement, the SBU said the operation, which used underwater “Sub Sea Baby” drones, was the first attack of its kind. Video shared by the SBU showed a large explosion at a port.

“As a result of the explosion, the submarine suffered critical damage and was effectively put out of action,” the SBU said. CNN could not independently verify the claim.

Russia acknowledged the Ukrainian attack but said it failed and that no ships or submarines were damaged.

“The enemy’s attempt to carry out sabotage using an unmanned underwater vehicle failed to achieve its goals,” state media quoted Alexei Rulev, head of the Black Sea Fleet press service, as saying. Rulev denied Ukrainian reports that a submarine was destroyed at the Novorossiysk naval base, Russian media reported.

The Kilo-class submarine is used to launch Kalibr cruise missiles, firing up to four at a time, according to the SBU statement. Russia has used the missiles throughout the war to strike Ukraine.

The agency added that the submarine is known as a “Black Hole” because of the hull’s ability to absorb sound and remain undetectable by sonar. It said that class of submarine costs around $400 million. Due to international sanctions, which have hampered Russia’s access to technological components, building a similar submarine could now cost up to $500 million, the SBU said.

The SBU said the vessel had been forced to remain in the port of Novorossiysk due to past successes of Ukraine’s sea drone operations, which forced Moscow to move many of its ships and submarines from Sevastopol Bay in the occupied Crimean Peninsula.

The attack Monday came amid a sustained bout of diplomacy to end Russia’s war in Ukraine. It was announced soon after the conclusion of a second day of talks between US and Ukrainian delegates in Berlin.

At a news conference Monday in Berlin, Ukrainian President Volodymyr Zelensky said that Kyiv must be “absolutely sure” about how its allies will guarantee its security before making any decisions about the front line in a potential peace settlement with Russia.

Speaking alongside German Chancellor Friedrich Merz, Zelensky said that any security guarantees should include monitoring the ceasefire.

“That is really the basis for security guarantees, because the question is: Who will carry out the monitoring? What sanctions will be applied if the monitoring missions are disrupted?” Zelensky said.

Although Zelensky said these questions have not yet been answered, Merz said the United States offered Ukraine “significant” guarantees in the talks in Berlin.

“What the US has offered here in terms of material and legal guarantees is really significant,” Merz said, without providing details.

After hosting European leaders later in the evening, Merz expressed optimism at the prospect of peace, posting to X: “For the first time since the war began, a ceasefire now seems possible.”

The European leaders agreed Monday evening on a commitment to provide a European-led multinational force, supported by the United States, as part of “robust security guarantees” under an agreement on ending Russia’s war in Ukraine.

In a joint statement following talks in Berlin, the leaders said the force would “assist in the regeneration of Ukraine’s forces, in securing Ukraine’s skies, and in supporting safer seas, including through operating inside Ukraine.”

In addition, they said, “a US-led ceasefire monitoring and verification mechanism” would be created with international participation “to provide early warning of any future attack” by Russian forces.

The European leaders committed to providing “sustained and significant” support to Ukraine to build its military and called for a “legally binding commitment” to restore peace in the event of a future attack, including through armed force and intelligence and logistical support.

The leaders of Britain, France, Germany, Denmark, Finland, Italy, the Netherlands, Norway, Poland, Sweden and the European Union stressed the need for security guarantees for Ukraine and said they would support whatever decisions Ukrainian President Volodymyr Zelensky makes on Ukrainian issues.

In a post Monday night on X, Zelensky praised his meeting Sunday with US President Donald Trump’s special envoy, Steve Witkoff, and son-in-law Jared Kushner. He suggested, without elaborating, that in a previous peace proposal he found “some things” to be “destructive.”

“It is important that they are no longer present in the new versions of the documents,” Zelensky wrote. “This matters, because dignity matters.” He noted that his country has a different position than Russia regarding Ukrainian territories, an issue that he said needs to be “discussed openly.”

“I believe that the American side, acting as a mediator, will propose various steps to try to find at least some form of consensus,” Zelensky said.

Daria Tarasova-Markina and Hira Humayun contributed to this report.