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

The Ukrainian armed forces' general staff said Russia had shelled several dozen positions on front lines on Friday but had no success capturing territory.

Russian forces failed in a bid to establish control over Ukraine's second biggest power plant at Vuhlehirska, north-east of Donetsk, and troops also tried to advance west from the city of Lysychansk but were pushed back, it said.

Russia's defense ministry did not immediately reply to a request for comment out-of-hours by Reuters.

Kyiv hopes that its gradually increasing supply of Western arms, such as US High Mobility Artillery Rocket System (HIMARS), will allow it to recapture lost territory.

Russia's defence ministry said on Friday its forces had destroyed four HIMARS systems between July 5-20, refuted by the United States and Ukraine. Reuters could not verify the battlefield reports.

The White House on Friday announced a package of additional support totalling around $270 million and said it was doing preliminary work on whether to send fighter aircraft to Kyiv, although such a move would not happen in the near term.

The 24 February invasion of Ukraine has caused Europe's biggest conflict since 1945, forcing millions to flee and turning entire cities to rubble.

The Kremlin says it is engaged in a "special military operation" to demilitarise and "denazify" Ukraine. Both Kyiv and Western nations say the war is an unprovoked act of aggression.

As the conflict drags on, credit rating firms Fitch and Scope downgraded Ukraine, two days after the country requested a debt payment freeze.

Grain deal

Friday's export deal hopes to avert famine among tens of millions of people in poorer nations by injecting more wheat, sunflower oil, fertilizer and other products into world markets, including for humanitarian needs, partly at lower prices.

A blockade of Ukrainian ports by Russia's Black Sea fleet, trapping tens of millions of tonnes of grain and stranding many ships, has worsened global supply chain bottlenecks and, along with Western sanctions, stoked food and energy price inflation.

Moscow has denied responsibility for the crisis, blaming instead sanctions for slowing its own food and fertilizer exports and Ukraine for mining the approaches to its Black Sea ports.

A UN official said a separate pact signed on Friday would smooth such Russian exports and that the United Nations welcomed US and European Union clarifications that their sanctions would not apply to their shipment.

Addressing Western concerns that reopening shipping lanes could leave Ukraine open to attack, Russian Defence Minister Sergei Shoigu said Moscow would not seek to take advantage of the de-mining of Ukraine's ports.

Kyiv does not see a risk of Russian ships attacking through the ports as they would be vulnerable to missile strikes, Ukraine's infrastructure minister Oleksandr Kubrakov said.

Senior UN officials said the deal was expected to be fully operational in a few weeks and would restore grain shipments from the three reopened ports to pre-war levels of 5 million tonnes a month.

Safe passage into and out of the ports would be guaranteed in what one official called a "de facto ceasefire" for the ships and facilities covered, they said, although the word "ceasefire" was not in the agreement text.

"Today, there is a beacon on the Black Sea. A beacon of hope... possibility... and relief in a world that needs it more than ever," UN Secretary General Antonio Guterres said.