Russia and Ukraine will sign a deal on Friday 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 defense and infrastructure ministers respectively to Istanbul to take part in a 1330 GMT signing ceremony, the two sides said.

U.N. Secretary-General Antonio Guterres, expected to co-sign the accord, and Turkish President 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.

Diplomats said last week that details of the plan included Ukrainian vessels guiding grain ships through mined port waters, with Turkey overseeing inspections of ships to allay Russian concerns they might smuggle weapons to Ukraine.

Some 20 million tonnes of grain are stuck in silos at Odesa, and dozens of ships have been stranded by Moscow's offensive.

Turkish Foreign Minister Mevlut Cavusoglu tweeted on Thursday that Friday's gathering in Istanbul would mark "the first step to solve the current food crisis".

'PACKAGE' DEAL

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

Moscow has denied responsibility for the worsening food crisis, blaming instead a chilling effect of Western sanctions for slowing its own food and fertiliser exports and Ukraine for mining the approaches to its Black Sea ports.

The United Nations and Turkey have been working for two months to broker what Guterres called a "package" deal - to restore Ukraine's Black Sea grain exports while easing Russian grain and fertilizer shipments.

Russian Foreign Ministry spokesperson Maria Zakharova said the European Union had proposed relaxing some earlier sanctions to shore up global food security. Moscow hoped this would create conditions for untrammeled exports of grain and fertilizers.

Turkey, a NATO member that has good relations with Russia and Ukraine alike, controls the straits leading into the Black Sea and has acted as a mediator on the grain issue.

Ukrainian President Volodymyr Zelensky met senior commanders on Thursday and said Kyiv's forces, now increasingly armed with precision, longer-range Western weaponry, had strong potential to turn the tide on the battlefield.

The United States believes Russia's military is sustaining hundreds of casualties a day, including thousands of officers up to general rank in total, in the course of the war, a senior U.S. defense official said on Friday.

"The chain of command is still struggling," the official said in Washington, speaking on condition of anonymity, although Ukraine had also endured significant casualties.

The official said Washington also believed that Ukraine had destroyed more than 100 "high-value" Russian targets in Ukraine, including command posts, ammunition depots, and air defense sites.

There have been no major breakthroughs on the front lines since Russian forces seized the last two Ukrainian-held cities in eastern Luhansk province in late June and early July.

WESTERN ARMS SUPPLIES

Russian forces are now focused on capturing all of neighboring Donetsk province on behalf of separatist proxies who have declared two breakaway mini-states covering the wider industrialized Donbas region.

In its morning update, Ukraine's general staff said Russian forces backed by heavy artillery kept trying to advance toward the cities of Kramatorsk and Bakhmut and the Vuhlehirska thermal power plant in Donetsk but made no notable progress.

Kyiv hopes that its gradually increasing supply of Western arms, such as the U.S. High Mobility Artillery Rocket System (HIMARS), will allow it to counter-attack and recapture lost eastern and southern territories.

Russia's defense ministry said on Friday its forces had destroyed four HIMARS systems between July 5-20. Kyiv denied the claims, calling them "fakes" meant to undermine the West's support for Ukraine. Reuters could not verify the assertions.

Ukraine has accused the Russians of intensifying missile and rocket strikes on cities in recent weeks in a deliberate attempt to terrorize its population.

Thousands of people have been killed and cities and towns devastated by Russian bombardment, with some far from front lines hit by missiles. Moscow denies deliberately firing on civilians and says all its targets are military.

However, there is a high chance of Russian longer-range weaponry missing their intended targets and causing civilian casualties because Moscow is increasingly using long-range air-defense systems to compensate for a shortage of ground-attack missiles, according to a British military intelligence update.

Russia says it is waging a "special military operation" to demilitarise its neighbor and rid it of dangerous nationalists.

Kyiv and the West say Russia is mounting an imperialist campaign to reconquer a pro-Western neighbor that broke free of Moscow's rule when the Soviet Union collapsed in 1991.