UN Secretary-General Antonio Guterres on Sunday expressed "deep concern" as Ukraine's maritime grain exports were halted after Russia suspended its participation in a deal that allowed the vital shipments.

"The Secretary-General is deeply concerned about the ongoing situation regarding the Black Sea Grain Initiative," his spokesman said.

"He has decided to delay his departure for the Arab League Summit in Algiers by a day to focus on the issue."

The July deal -- signed between Russia and Ukraine, and brokered by Turkey and the UN -- is critical to easing the global food crisis caused by the conflict.

Russia said it was halting its participation after accusing Kyiv of a "massive" drone attack on its Black Sea fleet, which Ukraine labeled a "false pretext".

"The Secretary-General continues to engage in intense contacts aiming at the end of the Russian suspension of its participation in the Black Sea Grain Initiative," Guterres's spokesman said.

"The same engagement also aims at the renewal and full implementation of the initiative to facilitate exports of food and fertilizer from Ukraine, as well as removing the remaining obstacles to the exports of Russian food and fertilizer."

EU foreign policy chief Josep Borrell said Sunday he and Guterres had spoken on coordinating action to keep exports flowing from Ukraine, adding "Russia must go back to agreement."

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