Russia Withdraws More Forces From Northeast Ukraine

Since the start of its offensive earlier this month, Ukraine has said it has retaken about 9,000 square kilometers, or about 3,500 square miles, of territory from Moscow’s forces in the region of Kharkiv, handing Russia the biggest setback since it invaded in February. That is more than one-tenth of all areas Russia gained and kept since the invasion.

The rapid gains have caught Moscow off guard, prompting criticism among supporters of the Kremlin about the way the military campaign is being handled.

Ukraine on Monday published photos of troops inspecting a warehouse filled with Russian arms and ammunition in Izyum, a city in southeastern Kharkiv that had for months served as a garrison for Russian forces.

In a report published early Monday, Ukraine said its armed forces had retaken 30 Russian-occupied settlements on Sunday alone, capitalizing on panic among Russian forces who have reportedly been fleeing the Kharkiv region and leaving equipment behind.

Sgt. Artur Shevtsov of the Dnipro-1 battalion, based near Slovyansk in eastern Ukraine, said the battalion’s sixth and seventh units had taken part in the battles for Izyum and Balakliya.

Sgt. Shevtsov said soldiers who participated had talked about seizing a vast amount of abandoned Russian arms and ammunition, carrying away anything they could find.

“We are taking advantage of our momentum,” he said, speaking on the phone from eastern Ukraine. “I don’t know what the leadership will decide after taking Kharkiv region, but we want to press the Russians as much as possible while we can.”

“Everyone is buoyed. We prepared a month for this,” he said of the offensive launched at the end of August. “And now it’s bringing results.”

The advance slowed Monday, as Ukrainians began to consolidate control and look for collaborators and pockets of Russian troops left behind. Kyiv now faces the challenge of finding the manpower and munitions needed to consolidate and eventually extend its gains.

Ukrainian President Volodymyr Zelensky said late Monday that Russian missile strikes against civilian infrastructure had cut power for hundreds of thousands of Ukrainians over the past few days as Russian troops pulled back.

“This is how they react to the defeat of Russian forces in the Kharkiv region,” Mr. Zelensky said.

Ukraine has signaled that it will make major new requests for weapons from the U.S. and its allies that the Kyiv government says it will need to press its offensive into 2023, according to a document shared with U.S. lawmakers and viewed by The Wall Street Journal.

Kyiv’s list of armaments desired include the Army Tactical Missile System, or ATACMS, which has a range of about 190 miles. The U.S. has declined to provide that system over concerns Ukraine could use it to strike Russian territory and spark a wider conflict with the West.

Senior Russian officials over the weekend conceded that the military operation in Ukraine was going badly, with pundits on state TV calling for a reassessment of Russia’s military strategy and an even harsher clampdown on dissent in Ukrainian cities and towns that Moscow still occupies. Ukraine’s offensive to push back Russian forces has raised questions even among pro-Kremlin propagandists who hew to the state-approved narrative.

“People who convinced President Putin that our special operation would be effective and brief, that we wouldn’t strike the civilian population, that we’d come and restore order, these people basically set us up,” said Boris Nadezhdin, a former Russian Duma deputy, appearing in a television talk show on Friday, the day Ukraine surged into the northern Kharkiv region.

Vitaly Ganchev, the head of the Russian-installed administration in the region, said Monday on Russian state-run news channel Rossiya 24 that Ukrainian forces vastly outnumbered Russian troops.

“If we talk about the force that was transferred to the counteroffensive of the Ukrainian army, it outnumbered our troops by about eight times, no less. Therefore, in order to preserve our personnel, it was decided to withdraw, to regroup,” he said.

In a video address on Sunday, Mr. Zelensky praised the country’s troops, volunteer fighters, front-line medics and intelligence officers, and urged them to continue their campaign to free Ukrainian territory.

“The path to victory is a difficult one,” he said. “But we are sure: You are capable of it. You will reach our border, all its sections. You will see our frontiers and the enemies’ backs. You will see the shining of the eyes of our people and of the occupiers’ heels. They will call it ‘goodwill gestures.’ We’ll call it a victory.”

The U.K.’s Defense Ministry said Monday that Russia had likely ordered the withdrawal of its troops from the Kharkiv region areas it had occupied for months in the face of Ukraine’s rapid advance, and said Russia’s defenses in the south were highly vulnerable too.

Russia had likely been forced to give priority to defensive actions across the front line amid a failure to properly resupply its forces in the southern Kherson region or bring in reinforcements needed to hold the line there. “The already limited trust deployed troops have in Russia’s senior military leadership is likely to deteriorate further,” the ministry said in a statement.

Mr. Zelensky noted the impression made internationally by Ukraine’s offensive. The success of the campaign could encourage Western partners to continue supplying the country with the arms and ammunition it needs to continue defending itself from Russia’s war of conquest.

“Today, your actions in the north, south and east of Ukraine are seen and noticed by all. The world is in awe. The enemy is in a panic,” Mr. Zelensky said in the video statement.

—Mauro Orru contributed to this article.

Write to Matthew Luxmoore at Matthew.Luxmoore@wsj.com