Ukraine has declared that it is mounting a counteroffensive to reclaim territory around the southern port city of Kherson, as Russia devotes the bulk of its forces to pounding eastern Ukraine and capturing Sievierodonetsk, the last Ukrainian controlled city in the Luhansk region.

Kherson was the first major city to fall as Russian forces swept north out of Crimea more than three months ago, and it has provided a key staging ground for Russian operations across southern Ukraine. In recent weeks, Russian forces — stretched thin and taking heavy losses as they gain ground in the eastern Donbas region — have concentrated their efforts in the south on fortifying defensive positions.

“Hold on Kherson,” the Ukrainian military said on Twitter on Sunday morning. “We’re coming.”

Ukrainian officials have for weeks telegraphed plans to launch a counteroffensive threatening Russia’s supply routes into Kherson on bridges over the Dnipro River. But they said the maneuver would require the delivery of Western artillery systems that had been promised by the United States and other allies.

It was not clear if new weapons were having an effect in the fight to reclaim territory around Kherson, but the Ukrainian military said Saturday evening that Russia had suffered losses as its forces were driven back to “unfavorable” positions around several villages and had been forced to call up reservists to serve as reinforcements, claims that could not be independently verified.

President Volodymyr Zelensky, speaking to the nation overnight, said that delivery of ever more powerful Western weapons was also vital in the “indescribably difficult” defense of the eastern Donbas region.

“Every day we are bringing closer the time when our army will surpass the occupiers technologically and by firepower,” he said. But that, ultimately, depends on continued and expanded Western support.

The Biden administration has approved sending long-range multiple launch rocket systems to Ukraine, a significant transfer that could hugely aid the country, U.S. officials said on Friday. Mr. Zelensky suggested an official announcement could come this week.

In other developments:

  • President Emmanuel Macron of France and Chancellor Olaf Scholz of Germany sought to revive diplomatic discussions during an 80-minute phone call on Saturday with President Vladimir V. Putin of Russia.

  • Ukraine’s defense minister, Oleksiy Reznikov, said that sophisticated Harpoon anti-ship cruise missiles had arrived. He said the Harpoons came courtesy of Denmark and would be used to try to break Russia’s Black Sea blockade and to protect the port city of Odesa.

  • The importance of long-range weapons systems is potentially decisive in the war.

  • Ukraine is holding on to its control of Sievierodonetsk, the last city it holds in the eastern Luhansk province, but Mr. Zelensky described the situation there as “indescribably difficult.”

  • Russia systematically uses thermobaric weapons in Ukraine.