(Reuters) - Expected negotiations between Russian President Vladimir Putin and Turkey's President Tayyip Erdogan remain the only hope to extend the Black Sea grain deal that is set to expire next week, Russia's RIA news agency reported on Monday.

The Black Sea deal, brokered between Russia and Ukraine by the United Nations and Turkey in July 2022, aimed to prevent a global food crisis by allowing Ukrainian grain trapped by Russia's invasion to be safely exported from Black Sea ports.

Citing an unnamed source familiar with negotiations, RIA reported "there is no optimism" for the extension of the deal - a position that Moscow has reiterated frequently in recent weeks.

"Our practice shows that it is the negotiations between the two leaders that are able to change the situation, the current difficult period is no exception," RIA cited the source as saying.

"Today, this remains the only hope."

Erdogan said on Saturday he was pressing Russia to extend the grain deal, currently due to expire on July 17, by at least three months and announced a visit by Putin in August.

The Kremlin said over the weekend there was no phone call scheduled and that there was no certainty about the two leaders meeting.

Ankara angered Moscow with its July 8 decision to release to Kyiv five detained Ukrainian commanders of a unit that for weeks defended a steel works in the Ukrainian city of Mariupol, with the Kremlin saying Ankara violated agreements.

(Reporting by Lidia Kelly in Melbourne; Editing by Lincoln Feast.)