KYIV, May 23 - Russia’s military said Tuesday it repelled what appeared to be one of the most serious cross-border attacks from Ukraine since the war began, The Associated Press reported.

Moscow blamed the raid that began Monday on Ukrainian military saboteurs. Kyiv portrayed it as an uprising against the Kremlin by Russian partisans. It was impossible to reconcile the two versions, to say with certainty who was behind the attack or to ascertain its aims.

The battle — which took place in southwest Russia’s Belgorod region, about 80 kilometers (45 miles) north of the city of Kharkiv, in eastern Ukraine — was a fresh reminder of how Russia itself remains vulnerable to attack, along with Russian-occupied regions of Ukraine.

The region is a Russian military hub holding fuel and ammunition depots and was included in Russian President Vladimir Putin’s order last year to increase the state of readiness for attacks and improve defenses.

Kremlin spokesman Dmitry Peskov refused to say how many attackers were involved in the assault or comment on why efforts to put down the attackers took so long.

Such cross-border attacks embarrass the Kremlin and highlight the struggles it faces in its bogged-down invasion of Ukraine.

Ukraine said Russian citizens belonging to murky groups called the Russian Volunteer Corps and the Freedom of Russia Legion were behind the assault.

Ukrainian Deputy Defense Minister Hanna Maliar said the attackers were Russian dissidents unhappy about Putin’s policies.

“These are Russian patriots, as we understand it. People who actually rebelled against the Putin regime,” she said.

The Freedom of Russia Legion said on Telegram the goal was to “liberate” the region.

The Russian Volunteer Corps implied on Telegram that the attack was over, adding: “One day, we’ll come to stay.” The post went up at around the same time as the Russian Defense Ministry claimed to have quashed the assault.

The U.K. Defense Ministry said it was “highly likely” that Russian security forces were fighting partisans in at least three locations in Belgorod.

“Russia is facing an increasingly serious multi-domain security threat in its border regions, with losses of combat aircraft, improvised explosive device attacks on rail lines and now direct partisan action,” it said Tuesday.

Belgorod officials earlier this year said they had spent nearly 10 billion rubles ($125 million; 116 million euros) on fortifications to protect the region.

The Russian Volunteer Corps claimed to have breached the border in early March. The shadowy group describes itself as “a volunteer formation fighting on Ukraine’s side.” It’s not clear if it — or the Freedom of Russia Legion — has any ties with the Ukrainian military. (ap/ez)