ANKARA, July 21 (AFP) - Ukraine and Russia were due on Friday (Jul 22) to sign an elusive deal designed to help relieve a global food crisis caused by blocked Black Sea grain exports.

The first major agreement between the warring sides since Russia's February invasion of its neighbour comes with global food prices soaring and people in some of the world's poorest countries facing starvation.

UN Secretary General Antonio Guterres was due to arrive in Turkey on Thursday for the signing ceremony at Istanbul's lavish Dolmabahce Palace on the Bosphorus Strait.

"The grain export agreement, critically important for global food security, will be signed in Istanbul (Friday) under the auspices of President (Recep Tayyip) Erdogan and UN Secretary General Mr Guterres together with Ukrainian and Russian delegations," the Turkish leader's spokesman Ibrahim Kalin tweeted.

Up to 25 million tonnes of wheat and other grain have been blocked in Ukrainian ports by Russian warships and landmines Kyiv has laid to avert a feared amphibious assault.