KYIV, 22 July (Reuters)- Russia and Ukraine will sign a deal on Friday (Jul 22) to reopen Ukrainian Black Sea ports for grain exports, Turkey and the United Nations said, raising hopes that an international food crisis aggravated by the Russian invasion can be eased.

Russia and Ukraine, both among the world's biggest exporters of food, sent their defence and infrastructure ministers respectively to Istanbul to take part in a 1.30pm GMT (9.30pm, Singapore time) signing ceremony, the two sides said.

UN Secretary-General Antonio Guterres, expected to co-sign the accord, and Turkish President Recep Tayyip Erdogan were to attend.

But fighting continued unabated in Ukraine's east and, underlining deep-seated enmity and mistrust, a Kyiv presidential adviser said it would sign no documents with Russia, rather only parallel deals on grain exports with the United Nations.

"In case of provocations, (there will be) an immediate military response" by Ukraine, Mykhailo Podoloyak tweeted.

The blockade by Russia's Black Sea fleet has worsened global supply chain disruptions and, along with Western sanctions imposed on Moscow, stoked high inflation in food and energy prices since Russian forces swept into Ukraine on Feb 24.

Full details of the accord were not immediately released. But Russian state news agency TASS, citing an unnamed source, said that three Ukrainian ports including the biggest export hub Odesa would be reopened.