ISTANBUL - Turkey said a deal it expects to be signed later on Friday with Russia, Ukraine and the United Nations for the resumption of Ukraine's grain exports will mark a first step to ease a global food crisis since Moscow's invasion.

Ukraine and Russia are among the world's top food exporters, and Ukraine's ports, including the major hub of Odesa, have been blockaded by Russia's Black Sea fleet.

The halt to grain exports during the five-month war has caused prices to rise dramatically, and reopening Ukrainian ports may potentially avert famine in parts of the world.

Some 20 million tonnes of grain are stuck in silos at Odesa, with dozens of ships stranded by Moscow's offensive, which it calls a "special military operation".

Russia denies the conflict has worsened the food crisis, instead blaming Western sanctions for slowing its own food and fertiliser exports and Ukraine for mining its Black Sea ports.

Turkish Foreign Minister Mevlut Cavusoglu said on Twitter on Thursday that Friday's meeting in Istanbul marks "the first step to solve the current food crisis".

Turkey would continue its efforts to resolve the conflict, Cavusoglu said. He also told state broadcaster TRT Haber that he did not see prospects for a ceasefire but that a deal on the grain exports could boost confidence between the two countries.

Turkey has said a general agreement was reached on a U.N.-led plan during talks in Istanbul last week and it will now be put in writing by the parties. Details of the agreement were not immediately known.

It is due to be signed at Istanbul's Dolmabahce Palace offices at 1330 GMT in a ceremony attended by U.N. Secretary General Antonio Guterres and Turkish President Tayyip Erdogan, Turkey's presidency said.

Russian Defence Minister Sergei Shoigu was travelling to Turkey for the grain deal, three sources told Reuters, and Ukraine's infrastructure minister would also attend.

There has been no confirmation from Ukraine or Russia. The Ukrainian Foreign Ministry said late on Thursday that another U.N.-led round of talks to unblock Ukraine's grain exports would take place in Turkey on Friday.

Turkish defence minister Hulusi Akar said last week that the deal to be signed includes joint controls for checking shipments in harbours. Turkey would also set up a coordination centre with Ukraine, Russia and the United Nations for grain exports.

Before the July 13 talks, diplomats said details of the plan included Ukrainian vessels guiding grain ships in and out through mined port waters; Russia agreeing to a truce while shipments move; and Turkey - supported by the United Nations - inspecting ships to allay Russian fears of weapons smuggling.

The United States welcomed the deal and said it was focusing on holding Russia accountable for implementing it.

The United Nations and Turkey, a NATO member and maritime neighbour to both Russia and Ukraine in the Black Sea, have been working for two months to broker what Guterres called a "package" deal - to resume Ukrainian Black Sea grain exports and facilitate Russian grain and fertiliser shipments.

After last week's talks in Istanbul, the United States sought to facilitate Russian food and fertiliser exports by reassuring banks, shipping and insurance companies that such transactions would not breach Washington's sanctions on Moscow.

(Reporting by Tuvan Gumrukcu in Ankara, Jonathan Spicer and Yesim Dikmen in Istanbul, Michelle Nichols in New York, Natalia Zinets in Kyiv, and Simon Lewis in Washington; Writing by Tuvan Gumrukcu; Editing by Dominic Evans and Alison Williams)