Russia on Monday claimed to have killed five Ukrainian soldiers inside its territory, the most serious allegation after a string of claims US officials said were fabricated pretexts meant to spark a broader conflict.
Ukraine vehemently denied the Russian report. "Not a single one of our soldiers has crossed the border with the Russian federation and not a single one has been killed today," its foreign ministry said, according to The Guardian's Luke Harding.
Russia and its state media outlets had accused Ukraine of a series of military incursions, mostly focused in the separatist regions of Luhansk and Donetsk.
US Secretary of State Anthony Blinken on Sunday said these were part of a "false flag" campaign to create an excuse to justify an invasion of Ukraine, which Russia has surrounded with some 150,000 troops.
The claim Monday was different in kind, alleging a more serious violation: that Ukraine had sent troops into Russia's own territory in the Rostov-on-Don region.
There was no independent proof that this occured, and Ukraine's foreign minister denied it in a tweet, writing: "No, Ukraine did NOT: Send saboteurs or APCs over the Russian border."
He also called on Russia to "stop your fake-producing factory now."
Russia's Southern Military District said in its release that five members of "a sabotage and reconnaissance group" were found on the Russian side of its border with Ukraine, and subsequently killed.
"On February 21, 2022, at about 06:00 Moscow time, in the area of the settlement of Mityakinskaya in the Rostov region, on the section of the state border of the Russian Federation with the Republic of Ukraine, the border detachment of the FSB of Russia discovered the penetration of a sabotage and reconnaissance group," the Southern Military District said.
"As a result of the clash, five violators of the border of the Russian Federation from the sabotage and reconnaissance group were destroyed."
The account was published bu Russia's RIA and TASS news agencies.
The US and Ukraine have said that they believe Russia was looking for a reason to justify invading Ukraine.
US officials waned that Russia may create fake videos that appear to prove its claims, and Insider identified examples which matched this warning over the weekend.
This story is developing. Please check back for updates.