Russia quit the agreement a week ago, saying that demands to improve its own food and fertilizer exports had not been met, and that not enough Ukraine grain had reached the poorest countries under the Black Sea deal.

"With the termination of the Black Sea Initiative, the most vulnerable will pay the highest price," Guterres told the U.N. Food Systems summit in Rome on Monday. "When food prices rise, everybody pays for it."

Since Russia quit the deal and began attacking Ukrainian food-exporting ports on the Black Sea and Danube river, global wheat and corn futures have risen sharply.

"This is especially devastating for vulnerable countries struggling to feed their people," Guterres said.

Guterres had written to Putin on July 11 in a final effort to save the deal. He proposed Russia extend it - with a daily limit of four ships traveling to Ukraine and four ships leaving - in return for connecting a subsidiary of Russia's Agricultural Bank, Rosselkhozbank, to the SWIFT global payments system.

A key demand by Moscow has been reconnecting Rosselkhozbank to SWIFT. The European Union cut it off in June 2022.

"I call on the Russian Federation to return to the implementation of the Black Sea Initiative, in line with my latest proposal," Guterres said. "I urge the global community to stand united for effective solutions in this essential effort."

The Black Sea grain deal was brokered by the U.N. and Turkey a year ago to combat a global food crisis worsened by Russia's February 2022 invasion of Ukraine. Ukraine and Russia are both leading grain exporters.

Russia's grain exports have risen since the war, but its ammonia and potassium-based fertilizers have fallen sharply.

"I remain committed to facilitating the unimpeded access to global markets for food products and fertilizers from both Ukraine and the Russian Federation, and to deliver the food security that every person deserves," Guterres said on Monday.