The U.S. military said a Russian fighter plane clipped the propeller of one of its spy drones and made it crash into the Black Sea on Tuesday in the first such direct encounter between the two powers since Russia invaded Ukraine over a year ago.

The Russian defence ministry offered a different account and Moscow's ambassador to Washington said his country viewed the incident involving a U.S. MQ-9 drone and Russian Su-27 fighter jet as a provocation.

The United States conducts regular surveillance flights in the region and has supported Ukraine with tens of billions of dollars in military aid, although it has not become directly engaged in the war.

Meanwhile, Ukrainian President Voldoymyr Zelenskiy said on Tuesday night that military commanders were unanimously in favour of defending the eastern frontline, including the ruined city of Bakhmut, which has been under siege by Russia for months.

"The main focus was on ... Bakhmut," Zelenskiy said in his nightly video address. "There was a clear position of the entire command: Strengthen this sector and destroy the occupiers to the maximum."

Zelenskiy dismissed three regional governors, including Luhansk in the east, Odesa on the Black Sea in the south and Khnelnitskiy region in the west, but no reason was given in the announcement by the government's parliamentary representative.

Ukrainian Deputy Defence Minister Hanna Malyar said Russia was "on the offensive" across the entire frontline in the eastern Donetsk region, with battles also raging around Kreminna, Bilohorivka and Sprine north of Bakhmut.

The defence of Bakhmut was important because a "large amount of enemy material is being destroyed ... A huge number of troops are being killed and as of today, the enemy's capacity to advance is being reduced", she told Ukrainian television.

On the diplomatic and economic fronts, talks continued to extend a deal to allow grain shipments from Ukraine's Black Sea ports that is due to expire this week, the United Nations and Turkey said. The Kyiv government rejected a Russian push for a 60-day renewal, half the term of the previous renewal.

DRONE CRASH

Two Russian Su-27 jets carried out what the U.S. military described as a reckless intercept of the American spy drone while flying in international air space. It said the Russian fighter jets dumped fuel on the MQ-9 - possibly trying to blind or damage it - and flew in front of it in unsafe manoeuvres.

After around 30 to 40 minutes, at 7:03 a.m. (0603 GMT), one of the jets then collided with the drone, causing it to crash, 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.

"This is a very sensitive stage in this conflict because it really is the first direct contact that the public knows about between the West and Russia," said Elisabeth Braw, a senior fellow at the American Enterprise Institute in Washington.

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