Ukraine’s Rapid Advance in East Puts Russian Forces on Defensive

After punching through Russian defenses in the northeastern Kharkiv region earlier this week, Ukrainian forces raced more than 30 miles toward Kupyansk, seizing the initiative after months of grinding combat.

Ukraine’s advances prompted Russia to rush reinforcements to Kharkiv and raised the prospect of the biggest reversal for Moscow since it was forced to withdraw forces from around the capital, Kyiv, in March.

Ukrainian President Volodymyr Zelensky said on Friday his country’s forces had retaken control of more than 30 towns and villages in the Kharkiv region in recent days and that measures were being taken to secure the gains. It couldn’t immediately be determined what reserves Ukraine has available to shore up the territory it seized and expand on its gains.

But images of the Ukrainian flag being raised over local government buildings and residents greeting soldiers with tears of joy have boosted morale across the country after months on the defensive. One video showed Ukrainian soldiers in the city of Balakliya clambering on top of an armored personnel carrier to tear down a banner reading, “We are with Russia! One people!”

“It is very difficult for us, but we are moving forward,” the commander of Ukraine’s armed forces, Lt. Gen. Valery Zaluzhny, said on the Defense Ministry’s Twitter account. An accompanying video showed civilians rejoicing at the return of Ukrainian forces after six months under Russian occupation.

The surprise offensive has put Ukrainian forces at the edge of Kupyansk, a city of some 30,000 before the war that is a critical rail and road hub for the resupply and movement of Russian occupation forces in eastern Ukraine. Control over the city would isolate Russian forces to the south in Izyum, which Moscow had sought to use as a staging ground for its own offensive.

The Russian occupation authorities said Friday that the situation in Kupyansk remained difficult but the military was able to hold the city, Russian state news agency RIA Novosti reported. They said no Ukrainian units have managed to penetrate into the city.

Vitaly Ganchev, the head of the Russian-installed administration in the Kharkiv region, said Ukrainian forces had scored a substantial victory in breaching Russia’s defenses in Kharkiv.

A video published by Russia’s Ministry of Defense on Friday showed the Russian military moving armored vehicles and personnel in the direction of Kharkiv.

North Atlantic Treaty Organization Secretary-General Jens Stoltenberg said the war was entering a critical phase as Ukraine makes progress on the battlefield.

Russia’s ambassador to the United Nations, Vasily Nebenzya, late Thursday dismissed reports of a breakthrough by Ukrainian troops during their counteroffensive, according to RIA Novosti. He said that the Ukrainian military managed to take only a few peripheral villages, and “there is no talk of any breakthrough.”

Russian military correspondents, however, said Russian forces were preparing to defend Kupyansk. Losing the city would severely degrade but not completely sever Russia’s ground lines of communication to Izyum, according to the Institute for the Study of War, a Washington-based think tank, predicting that Ukrainian forces would recapture it within days.

Despite the gains, U.S. Secretary of State Antony Blinken stressed Russia’s “significant resources” a day after meeting with Mr. Zelensky in Kyiv.

“Unfortunately, tragically, horrifically, President Putin has demonstrated that he will throw a lot of people into this at huge cost to Russia,” Mr. Blinken said. “This [war] is likely to go on for some significant period of time.”

Mr. Stoltenberg, speaking after meeting with Mr. Blinken in Brussels, pledged continued support for Kyiv.

Last week, Ukraine also launched an offensive in the southern Kherson region, but progress there has been slower.

Six months into the war, Kyiv has been under growing pressure to show Western partners it can convert billions of dollars of military support into meaningful gains on the battlefield.

The arrival of long-range precision artillery from the West helped slow Russia’s goal of capturing the whole of the eastern Donbas area in recent months. But Ukrainian forces hadn’t regained significant territory since their resistance around Kyiv compelled Russian forces to pull back from there in March.

In the east, Ukrainian forces had gradually ceded territory to Russian forces advancing behind a wall of artillery fire that laid waste to towns and villages.

In July, Ukraine said it was preparing an offensive to retake territory in Kherson, drawing Russian troops away from other stretches of the 1,100-mile front line. That left them stretched thinner in places including Kharkiv.

A video purportedly filmed in Balakliya showed tearful women embracing Ukrainian soldiers on their doorstep and offering them pancakes.

Photographs from another village retaken by Ukrainian forces in recent days showed the sign at the entrance to Shevchenkove had already been repainted in the yellow and blue of the Ukrainian flag.

“The cleanup is still ongoing,” said Oleh Synehubov, the head of the Kharkiv regional military administration. “The police and other services are returning to the liberated settlements. The work of state authorities is being resumed.”

Police urged residents of newly liberated areas to report crimes committed by occupying Russian forces. In villages occupied by Russia, posts in local chat groups urged residents to identify collaborators.

In the south and east, Ukrainian forces have retaken around 1,000 square kilometers, Mr. Zelensky said Thursday—about 385 square miles, more than the land area of New York City—since the start of September.

Presidential adviser Mykhailo Podolyak said the successes proved Ukraine could effectively use modern Western weaponry to de-occupy its territory.

The gains are “creating fissures within the Russian information space and eroding confidence in Russian command,” the Institute for the Study of War said.

Facing setbacks in the east, Russia has intensified strikes on Kharkiv, Ukraine’s second-largest city, in recent days. Mayor Ihor Terekhov said Russian artillery fire on Friday had wounded 10 people, damaging a kindergarten and private homes.

In the southeastern part of the country, Ukraine is considering shutting down the sole remaining reactor at the Zaporizhzhia nuclear plant, the International Atomic Energy Agency said Friday. Shelling overnight Thursday left the plant without a safe and sustainable source of backup power. The plant could turn to back up generators, but those only have enough fuel for about 10 days, according to Ukraine’s state-owned nuclear company, Energoatom.

“This is an unsustainable situation and is becoming increasingly precarious,” IAEA Director General Rafael Grossi said. “The power plant has no off-site power…. This is completely unacceptable. It cannot stand.”

Meanwhile, allied-nations armaments directors will meet Sept. 28 in Brussels to discuss how to increase ammunition production to support Ukraine, Bill LaPlante, the undersecretary of Defense for Acquisition and Sustainment, told reporters Friday.

The war in Ukraine has depleted American stocks of 155 mm ammunition and the Pentagon is looking at other allied partners to supply more as it works to increase production of the rounds, according to LaPlante.

As of Aug. 24, the U.S. military said it had provided Ukraine with up to 806,000 rounds of 155mm ammunition, which are used in howitzers, a key weaponry to Ukraine’s defenses.

“I think we are learning still about the global production of the 155,” Mr. LaPlante told reporters Friday.

—Georgi Kantchev, William Mauldin, Katia Rudeshko and Nancy Youssef contributed to this article.

Write to Isabel Coles at isabel.coles@wsj.com