Lysychansk, the final city in eastern Ukraine's Luhansk area that was still under Ukrainian control, has fallen to Russia.
On Sunday, the Ukrainian military claimed that it had been "forced to retreat" from the strategic city.
According to the country's Ministry of Defense, Russian Defense Minister Sergei Shoigu confirmed to President Vladimir Putin on Sunday that the military had seized Lysychansk and a number of adjacent communities.
In his nightly televised address to the country, Ukrainian President Volodymyr Zelensky stated that the withdrawal from Lysychansk was prompted by a desire to preserve the lives of Ukrainian forces.
"We will rebuild the walls, retake the territory, and protect the people above everything else," he declared.
Luhansk is one of the two areas that comprise Donbas, the region of eastern Ukraine where the conflict between Ukraine and separatists backed by Russia began in 2014.
After Putin's army failed to capture Kiev earlier this year, this region has become the focal point of his military ambitions in Ukraine.
The loss of Lysychansk brings Russia closer to its objective of capturing Donbas.
"After a hard battle for Lysychansk, the Ukrainian Troops were forced to withdraw from their occupied positions and lines," the General Staff of the Ukrainian Armed Forces tweeted on Sunday.
The proclamation was accompanied by a photograph that captioned, "Lysychansk: We'll be back."
In a Facebook post, the Ukrainian military acknowledged the superiority of Russian forces in terms of weapons, ammunition, and troops in the struggle for the city.
"We continue fighting. Unfortunately, determination and patriotism are insufficient for success; monetary and technical resources are also required," the post says.
In the region, pro-Russian separatists had already declared victory in the battle for Lysychansk.
The territory of Luhansk was declared "liberated" by Leonid Pasechnik, the leader of the self-proclaimed Luhansk People's Republic (LPR), which is supported by Russia.
"Today is the day we have finally done what we have all been working toward for the past eight years," Pasechnik stated.
He added that with the assistance of the Russian military forces, his troops liberated Lysychansk, completing the freedom of the Republic inside its historic borders.
Russia and Syria are the only nations that recognize the LPR's independence.
Saturday's video posted by the Russian state news agency Ria Novosti claimed to show locals installing the old Soviet Union flag at the Lysychansk Memorial to the Memory of the Fallen.
