Two Russian Su-27 jets carried out what the U.S. military described as a reckless intercept of the MQ-9 "Reaper" drone in international airspace before one of them collided with it at 7:03 a.m. (0603 GMT), causing the drone to crash into the sea.

Several times before the collision, the Russian fighter jets dumped fuel on the MQ-9, possibly trying to blind or damage it, and flew in front of the unmanned drone in unsafe manoeuvres, the U.S. military said.

Russia has not recovered the drone and the jet was likely damaged, the Pentagon said.

"In fact, this unsafe and unprofessional act by the Russians nearly caused both aircraft to crash," U.S. Air Force General James Hecker, who oversees the U.S. Air Force in the region, said in a statement.

Russia's defence ministry denied that its aircraft had come into contact with the unmanned aerial vehicle (UAV), which it said had crashed after "sharp manoeuvring". It said the drone had been detected near the Crimea peninsula, which Moscow annexed from Ukraine in 2014.

"The Russian fighters did not use their onboard weapons, did not come into contact with the UAV and returned safely to their home airfield," the defence ministry said.

The accounts of the incident in the Black Sea, which is bordered by Russia and Ukraine among other countries, could not be independently verified.

NATO's Supreme Allied Commander Europe, U.S. Army General Christopher Cavoli, briefed NATO allies about the first such episode since Russia's invasion of Ukraine in February 2022.

"We have been flying over that airspace consistently now for a year ... and we're going to continue to do that," said White House National Security Council spokesperson John Kirby.

The U.S. State Department later said it had summoned the Russian ambassador to protest what it called the downing of the drone.

EASTERN FRONTLINE FIGHTING

On the frontlines of eastern Ukraine, Russian troops pushed forward in waves and President Vladimir Putin reaffirmed his view that Russia's very existence as a state was at stake in the war.

In the eastern Donbas region, Russia and Ukraine are locked in the bloodiest infantry battle in Europe since World War Two after Moscow launched a winter offensive.

Putin has framed Moscow's year-long "special military operation" as a defensive pushback against what he sees as a hostile West bent on expanding into territories historically ruled by Russia.

"So for us this is not a geopolitical task, but a task of the survival of Russian statehood, creating conditions for the future development of the country and our children," Putin said during a visit to an aviation factory in Buryatia, some 4,400 km (2,750 miles) east of Moscow.

Putin accuses the West of using Ukraine as a tool to inflict "strategic defeat" on Russia. Ukraine and its Western allies say Moscow is waging an unprovoked war of conquest that has destroyed Ukrainian cities, killed thousands of people and forced millions more to flee their homes.

The Kremlin said Kyiv must accept "new realities" - its shorthand for Russia's claim to have annexed four regions, or nearly a fifth of Ukraine's territory.

"We have to achieve our goals. Right now this is only possible by military means due to the current position of the Kyiv regime," Russian state news agencies quoted Kremlin spokesman Dmitry Peskov as saying.

In a video address, President Volodymyr Zelenskiy said Ukraine's "future is being decided" in battles in the east, including the ruined city of Bakhmut. Fighting has raged there for eight months and Ukrainian commanders say they are killing enough Russian attackers to justify carrying on despite being almost surrounded.

Zelenskiy and his military chiefs agreed on Tuesday to keep defending Bakhmut despite concerns among some military analysts that the losses Ukraine is suffering could undermine its ability to mount a planned counter-offensive when the weather improves.

"It is key to the stability of the defence of the entire front," said General Valeriy Zaluzhnyi, commander in chief of the Ukrainian Armed Forces, praising his soldiers' fortitude.

'PUSHING HARD'

Further north on the frontline near Kreminna, Oleksandr, 50, commander of a unit in Ukraine's 110th battalion, said Russian assaults were still relentless despite having claimed little ground there.

"They are pushing hard. They are lobbing mortar bombs at us," Oleksandr told Reuters. He said Russian three-man fire teams advanced, with another wave following to replace them when they were killed.

"At night they always attack on foot and we sit, looking through our thermal goggles and shooting them."

Both sides reported more civilian casualties near the front.

Zelenskiy said six high-rise buildings were hit in the centre of Kramatorsk by a Russian missile, killing at least one person and wounding three. On the Russian-occupied side, in Volnovakha further south, the body of a woman lay on a street next to a ruined shop. A Russian military investigator said the area was hit by Ukrainian shells.

Off the battlefield, talks continued on Tuesday to extend a deal to allow grain shipments from Ukraine's Black Sea ports that is due to expire this week after Kyiv rejected a Russian push for a reduced 60-day renewal.