Russia's military said on Monday that troops and border guards had prevented a "diversionary reconnaissance" group from breaching Russia's border from Ukrainian territory and that five people had been killed, Russian news agencies reported.
"As a result of clashes, five people who violated the Russian border from a group of saboteurs were killed," the military said in a statement, adding the incident occurred near the village of Mityakinskaya in the Rostov region at 6 a.m. (3 a.m. GMT). Interfax cited the Russian military as saying that Ukrainian armed vehicles had been destroyed.
Ukraine rejected the report, calling it fake news, and said no Ukrainian forces were present in the Rostov region where the incident was alleged to have taken place.
"Not a single one of our soldiers has crossed the border with the Russian Federation, and not a single one has been killed today," Anton Gerashchenko, an official at Ukraine's interior ministry, told reporters.
The reported incident took place at a moment of extreme tension between the two countries as Russia continues to deny statements from the United States and its allies that it could invade Ukraine at any time.
Earlier on Monday, Ukraine dismissed a Russian claim that its forces had shelled a border post, denouncing it as "fake news" designed to inflame tensions.
Russia's Federal Security Service (FSB) had earlier alleged that a projectile fired from Ukraine had destroyed a border facility used by its guards.
Video released by the FSB showed a small, apparently, one-room shed with its roof and walls caved in and a Russian flag leaning against strewn debris. The claim increased concerns that Moscow is seeking a pretext to justify its military build-up on the border of Ukraine, which U.S. intelligence says is an invasion force. But Ukraine's spokesperson for its joint military operation against Russian-backed separatists in eastern Ukraine was unimpressed by the Russian allegation.
"They conduct different provocations and produce fake news every day, even a couple of times per day," Pavlo Kovalchuk told reporters in Kramatorsk.
"We couldn't stop them producing this fake news, but we always emphasise that we do not shoot at civilian infrastructure," he said.
"We don't use artillery to shoot back at occupation forces," he added, referring to the Moscow-backed rebels holding parts of the Donetsk and Lugansk regions.
Washington has repeatedly warned that Moscow is trying to justify a planned invasion by planting fake stories of Ukrainian aggression.
Russian-backed leaders in the rebel enclaves in Donetsk and Lugansk have ordered general mobilization and the evacuation of civilians, while shelling has intensified.
The FSB statement was released just as Russia's President Vladimir Putin was to chair an unscheduled meeting of his national security council.
"On February 21, at 9:50 am (6:50 a.m. GMT), an unidentified projectile fired from Ukraine completely destroyed a border facility used by the FSB border guard service in the Rostov region, around 150 meters (490 feet) from the Russian-Ukrainian border," it said.
The statement added that no one had been injured in the incident and that Russian military engineers had arrived at the scene.
Ukraine has been fighting separatists since 2014, when Russia annexed the Crimean peninsula after street protests in Kyiv overthrew a pro-Moscow president.
More than 14,000 people have already died in the fighting.
Kyiv's Western allies have warned in recent weeks of an escalation in the conflict with Russia massing more than 150,000 troops around Ukraine.
The southern Rostov region, where the FSB said the shelling took place, declared a state of emergency last week over an anticipated influx of civilians from the rebel-controlled territories.