Washington said the crash of a U.S. spy drone this week after an encounter with Russian jets had demonstrated Moscow’s “aggressive” behaviour, while Russia accused the United States of seeking to escalate tensions in the Black Sea region.

The drone incident on Tuesday was the first known direct U.S.-Russia encounter since Moscow launched its invasion of Ukraine on Feb. 24, 2022, and has laid bare the parlous state of relations between the world’s leading nuclear powers.

It came as Russia kept up its months-long drive to capture the small eastern Ukrainian city of Bakhmut, in what would be its first substantial victory in more than half a year. However, the Russian-installed leader of Ukraine’s eastern Donetsk region said on Thursday the situation around the now-ruined city remained “difficult” as Kyiv refused to withdraw its forces.

The U.S. and Russian defence ministers and military chiefs held rare phone conversations on Wednesday over the drone incident, in which the MQ-9 Reaper plunged into the Black Sea while on a reconnaissance mission in international airspace.

The U.S. military said two Russian Su-27 fighter planes had harassed the drone and sprayed fuel on it before one clipped the drone’s propeller, causing it to crash. Moscow said there was no collision and the drone crashed after making “sharp manoeuvres”, having “provocatively” flown close to Russian air space.

“There is a pattern of behaviour recently where there is a little bit more aggressive actions being conducted by the Russians,” said General Mark Milley, chairman of the U.S. Joint Chiefs of Staff, adding it was unclear whether the Russian pilots had intended to strike the drone.

Russian Defence Minister Sergei Shoigu told his U.S. counterpart that U.S. drone flights near Crimea’s coast “were provocative in nature” and could lead to “an escalation … in the Black Sea zone,” a ministry statement said.

Russia forcibly annexed Crimea from Ukraine in 2014.

Russia, the statement said, has “no interest” in escalation “but will in future react in due proportion” and the two countries should “act with a maximum of responsibility”, including by having military lines of communication in a crisis.

U.S. Defense Secretary Lloyd Austin declined to offer any details about his conversation with Shoigu, including whether he criticized the Russian intercept.

However Austin added: “The United States will continue to fly and to operate wherever international law allows. And it is incumbent on Russia to operate its military aircraft in a safe and professional manner.”

U.S. State Department spokesperson Ned Price, speaking to MSNBC, said the incident was most likely an unintentional act by Russia.

Russia has said the episode showed Washington was directly participating in the Ukraine war, something the West has taken pains to avoid.

“The Americans keep saying they’re not taking part in military operations. This is the latest confirmation that they are directly participating in these activities – in the war,” Kremlin Security Council Secretary Nikolai Patrushev said.

The United States has supported Ukraine with tens of billions of dollars in military aid but says its troops have not become directly engaged in the war, which Moscow portrays as a conflict against the combined might of the West.

Kyiv, for its part, said the drone crash showed Moscow was willing to expand the conflict zone to draw in other countries.

On the ground in Ukraine, Russia was making small advances near Bakhmut but at great cost, Milley said.

Ukraine’s President Volodymyr Zelenskiy said this week his military top brass had advised reinforcing Bakhmut.

Kyiv had appeared last month to be preparing to pull out of the city but has since decided to defend it, saying it is exhausting Russia’s attacking force there to pave the way for its own counter-attack.

“The situation in Artyomovsk remains complex and difficult,” Denis Pushilin, the Russian-installed head of Ukraine’s Donetsk region, said in an interview on state TV on Thursday, using the Soviet-era name for Bakhmut.

“That is, we do not see that there is any premise that the enemy is going to simply withdraw units,” he added.

In its daily intelligence update on the Ukraine war, Britain’s defence ministry said Russian attempts to capture the town of Vuhledar, about 150 km (93 miles) southwest of Bakhmut, had “almost certainly slowed” after repeated, very costly failed attacks over the last three months.

To the north of Bakhmut, Ukrainian troops in a bombed out village near the city of Kreminna battled to counter what they said was an attempt by Russia to undertake a giant pincer move.

“The Russians try to adapt in real time,” said a member of a drone unit call-signed “Zara”. “This makes great problems for us, because we have to think a couple of steps ahead – how do successfully complete the mission and not let the enemy know how we did it.”

Further south, in the Ukrainian-held town of Avdiivka, Donetsk Region police released video showing the evacuation of citizens, including 9-year-old Daryna and her parrot, Lemon.

Asked by a policewoman how long it had been since she had walked around in the town, Daryna said it had been 10 months.

“I dream for the war to end soon,” said Daryna, clad in a bright orange bulletproof vest and helmet.

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