Russia has said it would try to retrieve the wreckage of a US military drone that crashed over the Black Sea in a confrontation Washington blamed on two Russian fighter jets.
Moscow has denied that its Su-27 military aircraft had clipped the propeller of the unmanned Reaper drone, adding that it would react "proportionately" to any future US "provocations".
"Flights of American strategic unmanned aerial vehicles off the coast of Crimea are provocative in nature, which creates pre-conditions for an escalation of the situation in the Black Sea zone," the Russian defence ministry said.
However, Washington was unbowed, with Defense Secretary Lloyd Austin stressing the US would continue flying "wherever international law allows" following an apparent de-escalation call with his Russian counterpart.
Mr Austin called on Russia to "to operate its military aircraft in a safe and professional manner".
Russia confirmed the call and said Washington had initiated it.
Ukraine earlier suggested the incident over international waters was evidence the Kremlin wanted to draw the US into the conflict.
Russian Security Council secretary Nikolai Patrushev said in televised remarks Moscow would seek to retrieve the downed craft but was unsure if the effort would be a success.
"But it has to be done. And we will certainly work on it," he said.
The crash yesterday, which Washington said was the fault of reckless and unprofessional Russian conduct, further ratcheted up tension between Moscow and Western allies, already soaring over the Ukraine conflict.
Mr Patrushev said the incident was further proof that the US is a direct party to fighting between Moscow and Kyiv and said Russia had a responsibility to "defend our independence and our sovereignty".
Russia's defence ministry said it had scrambled jets after detecting a US drone over the Black Sea and denied causing the crash.
The Pentagon said the drone was on a routine mission when it was intercepted "in a reckless, environmentally unsound and unprofessional manner".
Russia said the aircraft had lost control, but White House national security spokesman John Kirby said the US "obviously" refuted the denial.
He added the US was trying to prevent the fallen drone from getting into the wrong hands.
"We've taken steps to protect our equities with respect to that particular drone -- that particular aircraft," Mr Kirby told CNN.
Russian intercepts over the Black Sea are common, Mr Kirby said, but this one was particularly "unsafe and unprofessional" and "reckless".
Ukraine said the incident was "provoked by Russia" and cautioned that it signalled President Vladimir Putin's aim to "expand the conflict".
"The purpose of this all-in tactic is to always be raising the stakes," Ukrainian National Security and Defence Council secretary Oleksiy Danilov said on social media.
The US uses MQ-9 Reapers for both surveillance and strikes and has long operated over the Black Sea keeping an eye on Russian naval forces.
Zelensky commits to holding Bakhmut
Elsewhere, Ukraine's President Volodymyr Zelensky repeated his commitment to defending Bakhmut, the small eastern city that has become the target of Europe's bloodiest infantry battle since World War II.
Moscow has waged a winter offensive involving hundreds of thousands of freshly called-up reservists and convicts recruited from jail as mercenaries.
It is trying to capture Bakhmut to secure its first substantial victory in more than half a year.
Kyiv had appeared likely last month to be preparing to pullout of the city but has since doubled down on defending it, saying it is exhausting Russia's attacking force there to pave the way for its own counter-attack later this year.
Mr Zelensky said in an overnight address that he had met his top military brass, and the main focus was on Bakhmut: "There was a clear position of the entire command: Strengthen this sector and destroy the occupiers to the maximum."
Some Western and Ukrainian military experts have questioned whether it makes sense for Kyiv to continue the battle for Bakhmut, because of its own heavy losses there.
Ukrainian Deputy Defence Minister Hanna Malyar said 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".
Intense fighting has also been under way north of Bakhmut, where Russia is trying to recapture territory it lost to a Ukrainian counter-offensive last year, and further south, where Moscow took heavy losses in failed assaults on the Ukrainian-held bastion of Vuhledar in February.
In the latest internal shakeup in Ukraine, Mr Zelensky dismissed the governors of three regions: Luhansk in the east, Odesa on the Black Sea in the south and Khmelnytskyi in the west. No reason was given.
He has replaced several other governors since the start of the year, including the leadership of most of the frontline provinces.
The front lines in Ukraine have barely moved for four months despite the most intense infantry battles of the war.
Russia's assaults have largely failed across most of the front line, apart from Bakhmut where it has captured the east of the city and advanced north and south as it tries to encircle it.
Both sides describe the fighting in Bakhmut as a "meat grinder", with the battlefield strewn with dead.
After recapturing swathes of territory in the second half of 2022, Ukraine has lately kept to the defensive, planning a counter-offensive later this year after muddy ground dries and Western armoured vehicles and tanks arrive.
Russia invaded its neighbour a year ago, describing Ukraine as a security threat.
It claims to have annexed nearly a fifth of Ukrainian territory. Kyiv and the West consider it an unprovoked war to seize land.
Tens of thousands of Ukrainian civilians and troops on both sides are believed to have been killed.
Ukrainian cities have been destroyed and millions of people have fled their homes.