KYIV — President Zelensky marked the 500th day of the war Saturday by hailing the country’s soldiers in a video from a Black Sea island that became the symbol of Ukraine’s resilience in the face of the Russian invasion.

Speaking from Snake Island, Mr. Zelensky honored the Ukrainian soldiers who fought for the island and all other defenders of the country, saying that reclaiming control of the island “is a great proof that Ukraine will regain every bit of its territory.”

“I want to thank — from here, from this place of victory — each of our soldiers for these 500 days,” Mr. Zelensky said. “Thank you to everyone who fights for Ukraine!”

It was unclear when the video was filmed. Mr. Zelensky was in Turkey on Saturday.

Russian forces took control of the tiny stone island on February 24, 2022, the day Moscow launched its invasion, in the apparent hope of using it as a staging ground for an assault on Ukraine’s biggest port and the headquarters of its navy, Odessa.

The island took on legendary significance for Ukraine’s resistance to the Russian invasion, when Ukrainian troops there reportedly received a demand from a Russian warship to surrender or be bombed. The answer supposedly came back, “Go (expletive) yourself.”

The island’s Ukrainian defenders were captured by the Russians but later freed as part of a prisoner exchange. After the island was taken, the Ukrainian military heavily bombarded the small Russian garrison there, forcing the Russians to pull back on June 30, 2022. 

The Russian retreat reduced the threat of a seaborne Russian attack on Odessa and helped pave the way for a deal to resume Ukrainian grain exports.

“Let the freedom that all our heroes of different times wanted for Ukraine and that must be won right now be a tribute to all those who gave their lives for Ukraine,” Mr. Zelensky said. “We will definitely win!”

Intense battles continued to rage Saturday in the country’s east and south as Ukrainian forces pressed their attacks against multi-layered Russian defenses in the initial stages of their counteroffensive.

The British Ministry of Defense said in its latest intelligence update that the eastern town of Bakhmut that was captured by the Russians in May has seen some of the most intense fighting along the front during the last week.

It said that Ukrainian forces have made steady gains to both the north and south of Bakhmut in the Donetsk region, noting that “Russian defenders are highly likely struggling with poor morale, a mix of disparate units and a limited ability to find and strike Ukrainian artillery.”

Amid the fighting, Russia and Ukraine accused each other of planning to sabotage the Russia-controlled Zaporizhzhia nuclear power plant, which is Europe’s largest, fueling fears of a radiation catastrophe. 

Ukraine’s military intelligence claimed Saturday that Russian troops have planted more mines around the plant, a claim that couldn’t be independently verified.

The head of the United Nations nuclear agency, Rafael Mariano Grossi, told the Associated Press Friday that the International Atomic Energy Agency experts had recently gained access to more of the site, including the cooling pond and fuel storage areas, and found no mines there. 

Mr. Grossi said he was still pushing for access to the rooftops of reactors where Ukrainian officials accused Russia of planting explosives.

As the tensions swirled around the plant, the Russian military has insisted that it has successfully fended off Ukrainian attacks in various sections of the front.

On Saturday, Russia’s defense minister, Sergei Shoigu, was shown visiting firing ranges where volunteer soldiers are being trained, a trip that comes two weeks after an abortive mutiny launched by mercenary chief Yevgeny Prigozhin, whose Wagner troops marched on Moscow in a bid to oust Mr. Shoigu.

Mr. Prigozhin agreed to end the mutiny, which represented the biggest threat to President Putin in his more than two decades in power, in exchange for an amnesty for himself and his troops and permission to move to Belarus. 

On Saturday, Russian messaging app channels ran comments by one of Wagner’s commanders, Anton Yelizarov, who said that the mercenaries have taken leave but would eventually deploy to Belarus.

Pitched battles along the front line in Ukraine are raging as NATO leaders are set to meet at Vilnius for a two-day summit next week to offer more help in modernizing Ukraine’s armed forces, create a new high-level forum for consultations, and reaffirm that it will join their alliance one day.

Ahead of the NATO summit, the U.S. has announced that it will provide Ukraine with cluster munitions, a move that President Biden described as a “difficult decision.” 

Two-thirds of NATO members have banned the munitions which have a track record of causing many civilian casualties, but America sees their delivery as a way to help bolster Ukraine’s offensive and push through Russian front lines.

Ukraine’s defense minister, Oleksii Reznikov, hailed the American move, saying that the delivery of cluster munitions will help the country de-occupy its territories while saving the lives of the Ukrainian soldiers.

Mr. Reznikov vowed that Ukraine will use the munitions only for the de-occupation of its territory and will not fire them at Russia’s proper territory. 

Mr. Reznikov also noted that the Ukrainian military will not use cluster munitions in urban areas to avoid hurting civilians, adding that they will be put to action in the field to “break through the enemy defense lines with minimum risk for the lives of our soldiers.”