A rare cross-border assault in southern Russia by anti-Kremlin fighters aligned with Ukraine stretched into a second day on Tuesday, with reports of an explosion at a defense factory and skirmishes at a crossing, in one of the most brazen incursions into Russian territory since the war began.
Russia’s Ministry of Defense said on Tuesday that it had pushed back all of the pro-Ukrainian fighters across the border from the region of Belgorod and that scores of “saboteurs” had been killed. The claim could not be verified, and people representing the anti-Kremlin fighters maintained that the attacks were continuing.
The Kremlin’s spokesman, Dmitri S. Peskov, described the attackers as “Ukrainian militants” whose violence justified Moscow’s war against its neighbor. “This once again confirms that Ukrainian militants are continuing their activities against our country,” Mr. Peskov told reporters on Tuesday.
When the incursions began on Monday, smoke could be seen billowing from explosions, according to drone video verified by The New York Times. Another video showed a soldier and an armored vehicle bearing Ukrainian markings about three miles into Russian territory. In Bryansk, a Russian border region to the north, a military factory warehouse near the town of Dyatkovo caught fire on Tuesday, local news media reported.
Some pro-Russian analysts feared that the attacks opened a new set of battlefield problems for Moscow.
Ukraine has denied any direct involvement in the incursions, casting the border attacks as a sign of internal division in Russia. A Ukrainian deputy defense minister, Hanna Maliar, described the fighters as “Russian patriots” rebelling against President Vladimir V. Putin’s government.
A group called the Free Russia Legion, made up of Russians who have taken up arms for Ukraine, claimed responsibility for taking the war to Russian territory. The volunteer unit operates under the umbrella of Ukraine’s International Legion, forces overseen by Ukrainian officers.
Ilya Ponomarev, an exiled former member of the Russian Parliament who described himself as the political representative of the legion, said by telephone on Tuesday that the incursions were an effort to force Moscow’s military to divert troops fighting in Ukraine and to destabilize Mr. Putin’s government by showing its inability to defend its long border with Ukraine.
“We think now they need to reconsider and deploy more forces all along the Ukrainian border,” Mr. Ponomarev said. He added that the group had captured about a dozen Russian border guards, a claim that could not be verified.
He also said Ukrainian officers had been aware of the operation but had not directed it.
A senior Ukrainian official said that Ukraine’s military was acting in support of the cross-border fighters and protecting Ukraine’s border in case of a Russian counterattack. The official, who spoke anonymously to reveal details about the mission inside Russia, said no Ukrainian fighters had entered Russian territory.
Andriy Zagorodnyuk, a former Ukrainian defense minister who now advises the Kyiv government, said the border incursions were a milestone because they involved armed troops, which could force Russia to deploy more of its forces along the border instead of the front line.
“Russians will see they have problems between their own citizens, so the idea of unified Russia is seriously damaged,” Mr. Zagorodnyuk said.
A British defense intelligence agency statement on Tuesday confirmed that fighting had “highly likely” broken out in three locations in the Belgorod region. It noted small-arms battles and drone strikes near Grayvoron, about six miles from the border, and said Russia had evacuated several villages.
Russia, it said, faces a growing security threat on the border with “losses of combat aircraft, improvised explosive device attacks on rail lines, and now direct partisan action.” It also said Moscow would most likely use the attacks to “support the official narrative that it is the victim in the war.”
On Monday, the Free Russia Legion said it had “liberated” the border village of Kozinka with another pro-Ukraine group called the Russian Volunteer Corps. Those claims could not be confirmed.
On Tuesday, Aleksey Baranovsky, a spokesman for the political wing of the Free Russia Legion, said the fighters had captured two more villages, Gorkovsky and Shchetinovka, and controlled about 7.7 square miles in Russia. Those claims also could not be confirmed.
A senior Ukrainian official who spoke on the condition of anonymity to discuss battlefield events acknowledged that the Free Russia Legion had suffered losses.
It would not be the first time that pro-Ukrainian fighters have attacked villages across the Russian border. In March, the Russian Volunteer Corps said it had staged a brief incursion into villages in the Russian region of Bryansk, and the Kremlin’s Security Council called an emergency session. The corps is led by a Russian nationalist in exile and is part of a motley collection of Russians who oppose Mr. Putin’s rule.
While residents of the Belgorod region have long been living with the sounds and explosions of the war, the attacks may deepen fear in Russia and dent Mr. Putin’s popularity, said Ivan Fomin, a Russian analyst with the Washington-based Center for European Policy Analysis.
“Some of the more hawkish segments of Russian society will see these attacks as another sign of the Kremlin’s weakness and incompetence,” he said. “So Putin can potentially lose some popularity among those who strongly support the war.”
But the incursion could also have a rally-round-the-flag effect, he said.
“If he can illustrate the infiltration of Russian territory by the sabotage groups from Ukraine,” Mr. Fomin said, “it might make it easier for him to sell a narrative about Russia being under attack and defending itself.”
Igor Girkin, a Russian military blogger also known as Igor Strelkov, wrote that if news of the border attacks were true, “then the inevitable creation of a continuous front along this border, which will have to be filled from somewhere with combined arms units and formations of the Russian Armed Forces, is on the agenda.”
Placing more soldiers along the border would stretch Russia’s forces even thinner and would be favorable to Ukraine, he concluded.
Even before Monday’s attack, Belgorod residents had shared a video, whose location could not immediately be independently confirmed, calling for the Russian government to arm them to defend against a possible incursion.
A man standing in front and reading from a paper says: “We fully understand that ahead of the offensive led by the Armed Forces of Ukraine, our forces won’t fully protect us. The front line is huge.”
When the assaults began on Monday, the Russian governor of Belgorod, Vyacheslav Gladkov, said Moscow’s military, border service and intelligence agency were “taking the necessary measures to eliminate the enemy.”
Mr. Gladkov put the region on a counterterrorism footing, establishing temporary restrictions of movement and suspending activities that involve dangerous substances.
He said that the region had been shelled 15 times on Tuesday morning and that one civilian had been killed. He later lifted the counterterrorism measures.
Pictures and videos verified by The Times appeared to show that the pro-Ukraine fighters had used at least three American-made armored vehicles during the incursion into Russia on Monday. It was unclear how they got access to American equipment. Russian forces captured at least two of the vehicles, visual evidence showed.
Matthew Miller, the State Department spokesman, said, “We’re skeptical at this time of the veracity of these reports.” He added that the United States does not “encourage or enable strikes inside of Russia.”
Russia’s border in the area is well fortified with mines, trenches and barriers. Since the war began, the authorities have spent about $125 million to strengthen the defenses of the Belgorod region, according to a statement by the regional construction minister in February.
But Russia, which claimed a significant military victory this week in the ruined city of Bakhmut after a grinding nine-month battle, has suffered several blows during the war. Those include an explosion that damaged the bridge linking occupied Crimea to the Russian mainland and the sinking of the cruiser Moskva, the flagship of the Black Sea fleet.
Yuriy Karin, an analyst with a group debunking Russian propaganda, said that after years of Russia’s denying its military interventions in eastern Ukraine, Ukraine may now be doing the same in southern Russia.
“It’s a mirror of the situation created by Russia in Crimea and Donbas” in 2014, when Russia sent in soldiers with unmarked uniforms and the Kremlin denied any affiliation with the fighters, Mr. Karin said.
Reporting was contributed by Oleksandr Chubko, Milana Mazaeva, Oleg Matsnev, Oleksandr Chubko, Julian E. Barnes, Riley Mellen, Christoph Koettl and Dmitriy Khavin.