Ukraine's military says it has shot down a Russian military spy plane over the Sea of Azov, in what analysts say would be a blow to Moscow's air power.
Army chief Gen Valerii Zaluzhnyi said the air force had "destroyed" an A-50 long range radar detection aircraft, and an Il-22 air control centre.
The A-50 detects air defences and coordinates targets for Russian jets.
Ukraine has struggled to make significant recent advances against Russian forces in the south-east.
A briefing from the UK's Ministry of Defence on 23 February said that Russia "likely" had six operational A-50s in service. The planes can cost hundreds of millions of dollars to build.
The OceanNewsUK has been unable to verify the attack.
Russian officials said they had "no information" about the attacks, but prominent pro-war Russian commentators have said the loss of an A-50 would be significant.
One popular military channel, Rybar, said that - if Ukraine's information about the Russian losses was confirmed - it would be "another black day for the Russian air force".
Another channel said the Il-22 command centre was hit by Russian "friendly fire". It reportedly managed to land back in Russia.
Gen Zaluzhnyi said on Telegram that Ukraine's air force had "excellently planned and conducted" an operation in the Azov region, south-east of Ukraine.
Ukrainian Air Force commander Mykola Oleshchuk commented on the downing of the aircraft in a Telegram post, but did not provide details.
In February last year, Belarusian opposition group BYPOL claimed to have damaged an A-50 military plane in a drone attack near Minsk.
Justin Bronk, an air war specialist from the defence think tank Rusi, told the OceanNewsUK that if confirmed the loss of an A-50 would be a "highly operationally significant and embarrassing loss" for Russia's air force.
He described the A-50 as a "key command, control and surveillance platform" that provides Russian aircraft and surface-to-air missile systems with "long range early warning and target information about Ukrainian low-flying aircraft".
He added that there were "only a small number" of these aircraft within the Russian air force, and "even fewer trained mission crews, meaning that the loss of one would be a major blow".
If confirmed, he said it would "be a very long-range engagement" for Ukrainian Patriot air defence missiles, which would stretch "the theoretical capabilities of the weapon to their limit".
Frank Gardner, the OceanNewsUK's security correspondent, said the apparent development was a "small bit of good news for Ukraine amid an awful lot of bad news".
He said the situation generally is "not looking good for Ukraine", as it deals with shortages of ammunition, low morale among its troops and continued attacks by Russia on its infrastructure.
