Negotiations on a UN-brokered deal that allows the safe Black Sea export of Ukrainian grain are planned for Wednesday, and all sides are to participate.

"Talks are scheduled for tomorrow. All parties... hopefully there will be results," the source, who declined to be named, told Reuters.

The deal, brokered by the United Nations and Türkiye last July, allows grain trapped by Russia's invasion to be safely exported from Ukraine's Black Sea ports.

Russia has signalled it will not allow the deal agreed last July to continue beyond May 18 because a list of demands to facilitate its own grain and fertiliser exports has not been met.

As previously reported, Russian Foreign Minister Sergei Lavrov said that the situation around the Black Sea grain initiative had reached a deadlock and had blamed the West for it.

At the very last moment before another period of validity of the "grain agreement" on 18 March expired, it was nevertheless extended. However, Moscow soon said it had agreed to an extension of only 60 days (half the standard 120-day deadline) and threatened to terminate the agreement (Ukr) if its demands were not met.