KRAMATORSK, UKRAINE (NYTIMES) - Ukrainian soldiers, seeking to spread Russian forces thin, launched a counteroffensive on Sunday (May 29) in Kherson, the key southern city that Moscow considered so securely under its thumb that it had introduced the rouble.

Ukraine's push in Kherson came as its forces were desperately battling to hold off Russia's efforts to conquer and cut off a strategic strip of eastern Ukraine that is central to Moscow's struggling war effort, and it had the effect of expanding the battlefield.

The opening of the new front underlined that when it comes to territory in Ukraine, little is for keeps as each side tries to exploit the enemy's shifting strategic vulnerabilities. That volatility promises to only increase as Ukraine receives more sophisticated long-range artillery and soon possibly US missiles.

On Sunday, President Volodymyr Zelensky of Ukraine, who has appealed for those weapons, sought to build morale by visiting the country's northeast, near Kharkiv, which is still under shelling.

He praised Ukraine's forces there for their success this month in pushing Russians back from the city's outskirts and said in an address on Sunday night that "Kharkiv suffered terrible blows from the occupiers".

The city, which suffered months of shelling that killed many civilians and forced tens of thousands to flee, was struck again hours after the Ukrainian president left, according to Reuters. But in the long term, Zelensky said late on Sunday, it is Russia that will pay the price.

"Russia has already lost not only the battle for Kharkiv, not only the battle for Kyiv and the north of our country," he said. "It lost its own future and any cultural ties to the free world. They all burned down."

The announcement of the Ukrainian counteroffensive - "Hold on, Kherson, we're coming!" the military said on Sunday morning on Twitter - signalled what may prove to be a new chapter in a war that has political, economic and humanitarian significance far beyond Ukraine's borders.

Kherson, a port city in Ukraine's agricultural heartland, was the first major city to fall as Russian forces swept north out of Crimea more than three months ago. After seizing it, Moscow used the city as a staging ground for operations across southern Ukraine.

But in recent weeks, Russian forces have concentrated their efforts in the south on fortifying defensive positions. Satellite images have shown Russians scrambling to build fortifications in Kherson, where the shoots of an insurgency surfaced this month.