Russian Military Response Questioned After Rare Assault Inside Borders

“The night was not entirely calm,” Mr. Gladkov wrote on Telegram on Wednesday morning, adding that houses, cars and office buildings in the city of Belgorod and other settlements had been damaged.

The Kremlin’s spokesman, Dmitri S. Peskov, said on Tuesday that Moscow was “deeply concerned” about the attack, while using the violence to try to further justify Russia’s 15-month-long invasion of Ukraine. However, he said that President Vladimir V. Putin would not be calling an emergency session of his Security Council.

The Security Council did meet after a shorter outbreak of violence on the border in Bryansk in March, when soldiers led by the same group briefly took control of a Russian village before being pushed back into Ukraine.

But some ardently pro-Russian voices openly expressed fear that the attacks in Belgorod would create new battlefield challenges for Russia, whose only significant military victory in the past nine months came in the last few days — claiming control of the ruins of the city of Bakhmut.

Igor Girkin, a military blogger and former Russian paramilitary commander in Ukraine, 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.”

The need to place more soldiers along the border, stretching the Russian forces even thinner, would be favorable for Ukraine, Mr. Girkin, who goes by Igor Strelkov, concluded.

Even before the attack that began on Monday, a group of Belgorod residents shared a video calling for the government to give them weapons to defend themselves from a possible incursion. The location of the video could not immediately be independently confirmed.

“Our city and region have long stopped being defended,” a man read from a paper, his hands trembling slightly as he stood in front of a group of other men. “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.”

While residents of the Belgorod region have long been living with the sounds of nearby explosions caused by the war, the attack over the past two days may deepen broader Russian fears and possibly even 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.”

However, depending on how the Kremlin and Russian state media portray the attacks, the incursion could also have a rally-round-the-flag effect, Mr. Fomin said.

“For now, Putin has had difficulties explaining why he started this war, what are its goals, and why Russians should go risk their lives in Ukraine,” he said. “But if he can illustrate the infiltration of Russian territory by the sabotage groups from Ukraine, it might make it easier for him to sell a narrative about Russia being under attack and defending itself.”

Mr. Peskov sought to tamp down talk of Russians taking up arms against fellow citizens, saying that the fighters were Ukrainians, not Russians. But the group that claimed the attack said that it was composed of Russians who “have finally returned home,” as they put it on Telegram.

Yuriy Karin, an analyst with Information Resistance, a group that debunks Russian propaganda, said that Russia’s halting official response to the attack showed “shock” that an incursion had been perpetrated.

“Russian propaganda was in denial,” he said.

Andrew E. Kramer contributed reporting.