The secretary-general of the United Nations has raised his concerns regarding the implementation of the Black Sea Grain Initiative, amid ongoing threats by Russia not to renew the pact.

The move comes after the U.N. said no ships were inspected on Tuesday under the deal "as the parties needed more time to reach an agreement on operational priorities." Inspections resumed on Wednesday.

In a fresh warning, Russia said Thursday there would be no extension of the landmark wartime Black Sea grain deal beyond May 18 unless the West removes a series of obstacles to its grain and fertilizer exports.

Brokered by the U.N. and Türkiye in July last year, the grain agreement has given Ukrainian grain shipments a safe corridor in the Black Sea after they were blocked amid Russia's invasion.

The initiative sought to help alleviate a global food crisis worsened by the conflict, disrupting exports from two of the world's leading grain suppliers.

"Without progress on solving five systemic problems ... there is no need to talk about the further extension of the Black Sea initiative after May 18," the Russian Foreign Ministry said in a statement.

Moscow has repeatedly threatened to walk away from the agreement if obstacles to its grain and fertilizer exports remained.