The Russian government has claimed its forces repelled a cross-border attack from Ukraine, killing more than 70 "saboteurs." A Ukrainian official has denied any involvement.
The governor of the western Russian region of Belgorod bordering Ukraine said on Monday fighting broke out after an incursion by a Ukrainian sabotage group. The governor said a man and a woman died in the battle.
Russian investigators are calling it an act of terrorism. Russia's defense ministry claimed to have killed more than 70 attackers.
The administration of Russian President Vladimir Putin is believed to be taking the situation seriously. State-run Tass news agency reported that the Russian Security Council may take up the issue when it meets this week.
Two groups have claimed responsibility for the attack on social media. They describe themselves as Russians opposed to their country's invasion of Ukraine.
Roman Kostenko, a high-ranking Ukrainian military officer, told NHK on Tuesday that these groups are working in partnership with his country and may have been behind the attack. But he stressed that they have their own goals and Ukraine has no intention of claiming Russian territory.