Ukrainian forces launched the most “massive” drone attack “in the history of the conflict” at the headquarters of the Kremlin’s Black Sea fleet in Crimea, according to Russian officials, who said Saturday they would no longer participate in a United Nations-brokered deal to ship grain and other products from Ukrainian ports in retaliation.

Russia’s 410-foot naval flagship, The Admiral Makarov, appeared to be damaged in the late Friday attack at Sevastopol Bay, in which explosions rocked the heavily fortified navy ship yard, according to the Sunday Times of London. A minesweeper was also slightly damaged, the outlet reported.

“Today at night, the most massive attack by UAVs (unmanned aerial vehicles) and remote-controlled surface vehicles in the waters of the Sevastopol bay was undertaken,” said Russia-installed Crimea governor Mikhael Razojaev in an interview with Russia’s TASS news agency.

The extent of the damage — some captured on video footage showing fires and smoke at several locations around the naval base– could not be verified.

The Russian navy had earlier repelled a drone attack in which all of the devices had been “shot down,” Razojaev said on Telegram.

The late Friday attack — part of a major counteroffensive by Ukraine to regain territory annexed by Russia in the southern part of the country — caused minor damage to the Black Sea port fleet, which was involved in protecting convoys exporting grain from Ukraine, Razojaev said.

As a result of the attack, Russian officials said they had suspended participation in the deal to allow the secure export of grain and other agricultural products from Ukraine’s Black Sea ports, TASS reported Saturday.

Under the July agreement, Ukraine was able to restart its grain and fertilizer exports which were halted when Russia invaded the country on Feb. 24. The deal has seen more than nine million tons of grain exported from Ukraine and has brought down food prices around the world.

The Russian suspension comes one day after Antonio Guterres, leader of the UN, urged Russia and Ukraine to renew the agreement on the eve of its 120-day deadline.

The deal was set to expire Nov. 19. Guterres also called upon countries in the West to help remove obstacles blocking Russian grain and fertilizer exports.

Meanwhile, Russian officials said Britain was also to blame for the drone attack on its fleet at Sevastopol.

“The preparation of this terrorist act and the training of military personnel of the 73rd Ukrainian special maritime operations center were carried out by British specialists based in Ochakov, Mykolaiv region of Ukraine,” the Russian defense ministry said Saturday on Telegram.

Officials in the United Kingdom denied that their forces had aided in the attack, adding that “Russia is peddling false claims of an epic scale.”

Ukrainian authorities said Russia’s armed forces had launched “a fictitious terror attack” on its own ship in an effort to “blackmail” Ukraine by withdrawing from the grain deal.

Ukrainian foreign minister, Dmytro Kuleba said Russia was using “a false pretext” to block the grain shipments, which are among Ukraine’s most important exports.

Also Saturday, Russia’s mercenary Wagner Group called on experts to generate “new ideas” to improve their chances of winning the war.

Wagner Group’s Yevgeny Prigozhin made the announcement on Vkontakte, a Russian social media platform.

Prigozhin, who is close to Russian leader Vladimir Putin, “will provide a comfortable environment for generating new ideas in order to increase the defense capability of Russia, including in the information field. If the project shows its success and relevance, we will consider the need to open more branches,” the statement read.