KYIV, Ukraine >> Ukrainian forces pressed deeper into Russia today, trying to capitalize on their surprise cross-border offensive, as Moscow moved quickly to shore up its defenses against the largest assault on Russian soil since the war began.

After capturing several small settlements in the last few days, Ukraine was battling to take full control of a town near the border and sending small units to conduct raids farther into the southwestern Russian region of Kursk.

At the same time, the Russian military announced that it was sending more troops and armored vehicles to try to repel the attack. Russian television released videos of columns of military trucks carrying artillery pieces, heavy machine guns and tanks.

Perhaps preparing for retaliation, Ukrainian authorities today said that they were evacuating 20,000 people from the Sumy region, which is across the border from Kursk.

The flurry of movement underscored the extent to which Ukraine’s attack had introduced an unpredictable new element into a war that has been progressing in slow motion elsewhere on the battlefront. Ukraine’s success so far could provide a much-needed boost to a country whose forces have been steadily losing ground for many months, while bringing the war home to Russian civilians in a more serious way.

But military analysts have questioned whether the operation is worth the risk, given that Ukrainian forces are already stretched. It is also not clear whether the mission will help Ukraine improve its position on the rest of the battlefield by forcing Russia to divert troops from elsewhere to reinforce the border region.

The Ukrainian military has enforced a policy of silence about the operation, and it has not publicly acknowledged launching a cross-border attack.

Ukraine’s allies in the past have been wary of Ukrainian incursions in Russia, fearing that it could escalate the war, but there have been no public indications from Western capitals that they oppose the assault. The United States has said the Ukrainian incursion does not violate U.S. guidance.

However, senior U.S. officials have said privately that they did not get a heads-up about the operation and were still seeking clarity about its logic and rationale.

The officials said that they understood Ukraine’s need to change the optics and the narrative of the war, but that they were skeptical Ukraine could hold the territory long enough to force Russia to divert significant forces from the offensives it is pressing in eastern and southern Ukraine.

“It’s a gamble,” said one senior administration official.

Still, Mykhailo Podolyak, a top Ukrainian presidential adviser, was upbeat about the international response. “Most quietly approve,” he wrote on social media Thursday evening, adding that a significant part of the world now considers Russia “a legitimate target for any operations and types of weapons.”

The fighting in Russia showed no signs of abating today, with the Ukrainian military saying that it had struck a Russian airfield in the Lipetsk region, which borders Kursk, hitting warehouses that contain guided aerial bombs. Local Russian authorities said a large drone attack had caused several explosions and that a fire had broken out at a military airfield.

Ukrainian authorities also said a Russian strike on a supermarket in Kostiantynivka, an eastern Ukrainian town 200 miles south of the area of the fighting, killed 14 people and wounded 43 others. The claims from both sides could not be independently verified.

Military analysts said the attack across the border had involved elements of at least four brigades in a rare example of successful maneuver operations involving support from artillery, air defenses and electronic warfare, resulting in quick advances on the ground.

“It seems to be a fairly well-coordinated and planned combined armed operation,” said Franz-Stefan Gady, a Vienna-based military analyst. “You have electronic warfare assets that were deployed to jam Russian command and control. You have air defenses that were moved in to create air defense bubbles around the Ukrainian advance. And then you have fairly effective mechanized formations moving forward at a fairly steady pace.”

Gady and other experts said the main question now is whether Ukraine can maintain the momentum and turn the success on Russian territory into useful gains. The Ukrainian army has few reserves it can pour into the fight, and it continues to suffer from shortages of weapons and ammunition, analysts say.

It also remains unclear what Ukraine ultimately hopes to accomplish. A senior Ukrainian official who spoke on condition of anonymity to discuss the operation said the goal was to draw Russian troops away from other parts of the front line where Ukrainian units are struggling. But military experts said Russia would likely be able to respond with reserves who were not fighting in Ukraine.

“Does it really solve any of the larger military strategic problems that the other parts of the front line are suffering from?” Gady asked.

Mick Ryan, a retired Australian army major general and a fellow at the Lowy Institute, a research group, said one objective may be to boost morale in the Ukrainian population. “Given the past eight months of defensive operations, constant aerial attacks on infrastructure and ongoing power shortages, the will of the people will be at the forefront of the Ukrainian government’s considerations about the trajectory of the war,” he said.

A map of the battlefield by the Black Bird Group, a Finland-based organization that analyzes images from the battlefield, shows that Ukrainian troops have gained about 100 square miles of Russian territory since the beginning of the attack, although it remains unclear whether they have secured control of all of it. They have advanced past two lines of Russian defenses.

In particular, the Ukrainian army has entered Sudzha, a small town of about 6,000 people 6 miles from the Ukrainian-Russian border. Ukrainian troops today claimed in a video that the town was under their control. The claim could not be independently verified.

Emil Kastehelmi, an analyst from the Black Bird Group, wrote on social media that some Ukrainian units appeared to be conducting probing raids farther north in the direction Lgov, a town about 50 miles from the border, in what appears to be a test of Russian defenses.

A video posted on social media this morning and verified by The New York Times showed a column of destroyed Russian military vehicles just east of Rylsk, a Russian town west of the border area captured by Ukraine.

It remains to be seen whether Ukraine will try to push farther into Russian territory to solidify control over the area it has captured, or retreat after a few days, as has happened in previous, smaller-scale cross-border raids.

Kastehelmi said Ukraine could not continue farther north without widening its flanks and exposing itself to Russian counterattacks. “Time is also running against Ukrainians,” he wrote. “Russians won’t be disorganized forever.”

This article originally appeared in The New York Times.

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