KYIV, July 23 (Reuters) - The United States promised more military support for Ukraine, including drones, and is doing preliminary work on whether to send fighter aircraft, as fighting raged on in the east of the country five months into Russia's invasion.

Moscow and Kyiv signed a landmark deal on Friday to unblock grain exports from Black Sea ports. However, representatives declined to sit at the same table and avoided shaking hands at the agreement ceremony in Istanbul, reflecting wider enmity.

Ukrainian President Volodymyr Zelenskiy hailed Friday's agreement as unlocking around $10 billion worth of grain exports, needed to ease a food crisis.

But on the war, he said there could be no ceasefire unless lost territory was retaken.

"Freezing the conflict with the Russian Federation means a pause that gives the Russian Federation a break for rest," he told the Wall Street Journal.

"Society believes that all the territories must be liberated first, and then we can negotiate about what to do and how we could live in the centuries ahead."

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 U.S. 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 Feb. 24 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.