A crucial deal allowing the export of grain from Ukraine is set to expire Monday unless Russia agrees to an extension.

The last ship to travel under the Black Sea deal left the port of Odesa early on Sunday, Reuters reported.

Russian President Vladimir Putin told South African President Cyril Ramaphosa in a phone conversation on Saturday that the deal had not met its objectives

"[Putin] stressed that the obligations set out in the relevant Russia-UN memorandum to remove obstacles to the export of Russian food and fertilizers still remain unfulfilled," the Kremlin readout of the call said.

"Moreover, the main goal of the deal, namely the supply of grain to countries in need, including those on the African continent, has not been realized," it said.

Putin has previously complained about restrictions over its own exports but has agreed to previous extensions.

The deal, which was first brokered by Turkey and the United Nations following the outbreak of Russia's war on Ukraine in July 2022, creates procedures to ensure the safe export of grain from Ukrainian ports.

Under the deal, grain ships are able to navigate through a safe corridor in the Black Sea under the direction of Ukrainian pilots, and then pass through the Bosphorus Strait -- an important shipping corridor in north-west Turkey -- in order to reach global markets.

It has proven vital for stabilizing global food prices and bringing relief to the developing countries who rely on Ukrainian exports.

The deal's survival continues to rest in Russia's hands, and Moscow has repeatedly threatened to quit the pact.

Prior to its third renewal in May, the Kremlin had remained elusive as to whether it would remain in the deal, before agreeing to a last-minute extension.

Russia also suspended its participation for a few days in late October and early November 2022, citing drone attacks on the city of Sevastopol in occupied Crimea.

Moscow later announced that it would reverse course and rejoin the agreement for a further 120 days, following mediation.