The head of the United Nations said he was “deeply concerned” by Russia’s decision to suspend its participation in a UN-brokered grain deal and delayed a foreign visit to try to revive the agreement, which aims to ease a global food crisis.

Moscow halted its role in the Black Sea deal on Saturday, effectively cutting shipments from Ukraine, one of the world’s top grain exporters. It said it was responding to what it called a major Ukrainian drone attack earlier in the day on its fleet near the port of Sevastopol in Russian-annexed Crimea.

UN secretary-general Antonio Guterres was engaging in “intense contacts aiming at the end of the Russian suspension of its participation”, UN spokesperson Stephane Dujarric said. Guterres had delayed his departure for the Arab League Summit in Algiers for a day to focus on the issue, his statement added.

Nato said Ukraine’s grain exports had helped reduce food prices the world over. We call on Russia to reconsider its decision and renew the deal urgently, enabling food to reach those who need it most,” spokesperson Oana Lungescu said.

Ukraine’s infrastructure ministry said 218 vessels were “effectively blocked” by the decision. Wheat prices on international commodities markets were expected to leap on Monday as a result, analysts said.

The European Union also urged Moscow to reverse course. “Russia’s decision to suspend participation in the Black Sea deal puts at risk the main export route of much needed grain and fertilisers to address the global food crisis caused by its war against Ukraine,” EU foreign policy chief Josep Borrell said on Twitter.

Turkey negotiated the July 22 grain deal along with the United Nations. Its defence ministry said minister Hulusi Akar was in talks with Russian and Ukrainian counterparts to resume the agreement and had asked parties to avoid any “provocation”.

On Saturday, US President Joe Biden called Russia’s move “purely outrageous”, saying it would increase starvation, while Secretary of State Antony Blinken accused Moscow of weaponising food. On Sunday, Russia’s ambassador to Washington, snapped back, saying the US response was “outrageous” and made false assertions about Moscow’s move.

Russia’s defence ministry said Ukraine attacked the Black Sea Fleet near Sevastopol with 16 drones early on Saturday, and that British navy “specialists” had helped coordinate what it called a terrorist attack. Britain denied the claim.

Ukraine has neither denied nor confirmed it was behind the attack while Ukrainian military suggested that Russians themselves might have been responsible for the explosions.

Russia said it had repelled the attack but that the ships targeted were involved in ensuring the grain corridor out of Ukraine’s Black Sea ports.

Ukrainian foreign minister Dmytro Kuleba said Moscow used the explosions 220km (137 miles) away from the grain corridor as a “false pretext” for a long-intended move.

“Russia took the decision to resume its hunger games long ago and now tries to justify it,” Kuleba said on Twitter, without offering any evidence.

President Volodymyr Zelensky’s chief of staff accused Russia on Saturday of inventing attacks on its own facilities.

Kyiv often accuses Russia of using the Black Sea Fleet to fire cruise missiles at Ukrainian civilian targets, a charge supported by some military analysts, who say that makes the fleet a legitimate military target.

Russia’s departure from the grain deal marks a new development in an eight-month war that began with Russia’s invasion in February.