Talks on a U.N-brokered deal that allows the safe Black Sea export of Ukrainian grain are scheduled for Wednesday, with all sides in the negotiations involved, a senior Ukrainian source said on Tuesday.

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

Russia, which invaded Ukraine in February last year, 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 fertilizer exports has not been met. (Reporting by Pavel Polityuk, Editing by Timothy Heritage)