Ukraine announced a military offensive Monday in areas across its south, signaling the launch of a long-awaited push to retake Russian-held territory.
"Today, we started offensive actions in different directions," Natalia Humeniuk, a spokesperson for the southern military command, told Ukraine's public broadcaster Suspilne.
The apparent start of the major counteroffensive comes after weeks of buildup and nearly six months since Russian forces seized swaths of territory in southern Ukraine in the early days of the war.
Kyiv has long vowed to retake that land and has displayed growing confidence in recent weeks, increasingly taking the initiative in a conflict that the Kremlin itself has admitted was stalled.
Military observers have been anticipating a counterattack on the ground in the south as Ukraine seemingly targeted Russian military sites, as well as key bridges, with strikes deep behind defensive lines there and in nearby Crimea, which Moscow annexed in 2014.
The front lines in the south are centered on Kherson, a region Russia seized in its entirety in March. A strategic gateway to the Black Sea and Crimea, it has been critical in cementing Moscow's grasp on the area.
Early reports are that Ukrainian forces have broken through the first set of Russian defenses in places around Kherson.Neil Melvindirector of international security studies, Royal United Services Institute
The extent of the Ukrainian offensive and any gains made were unclear as social media buzzed with unconfirmed reports.
Humeniuk expressed caution and called for patience, noting that the Russian military was still strong and had been reinforcing its positions in the area.
"We do not declare the beginning of the offensive or the end of the offensive," she said in a clarifying comment by phone. "But under the pressure of our actions, the enemy began to retreat. It is currently recorded that the enemy has withdrawn from some of its positions."
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NBC News has not verified the claim.
"Early reports are that Ukrainian forces have broken through the first set of Russian defenses in places around Kherson," said Neil Melvin, the director of international security studies at the Royal United Services Institute, a London-based think tank.
"These were likely defended by some of the weaker Russian forces, notably from the occupied eastern areas of Ukraine, and the real test of Ukraine's ability to turn back the Russian forces is still likely to lie ahead," he said.
'Major offensive effort,' White House says
Regarding the current operations, White House spokesman John Kirby said, "There have been efforts throughout the six months of this war where Ukraine has gone on the offense. This is a major offensive effort but the idea of being on the offense is not new to Ukrainians." He declined to comment on future Ukrainian operations.
Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelenskyy, appeared to make a veiled reference to the news. "The only possible option for negotiations with Russia is being conducted by a special Ukrainian delegation in the southern and other directions of the front-line," he said in a tweet.
But Kirill Stremousov, the deputy head of the Moscow-appointed administration for the Kherson region, denied the reports of a Ukrainian offensive, calling them a "fantasy" and adding that the region's capital was leading "a peaceful life," the Russian state news agency Tass reported Monday.
Russian media also reported Ukrainian strikes on the occupied towns of Nova Kakhovka and Beryslav near the city of Kherson overnight.
Meanwhile, the governor of the nearby Mykolaiv region, Vitaliy Kim, said at least two people were killed and 11 injured in what he described were heavy Russian strikes on the city of Mykolaiv, which is under Ukraine's control.
It's a bit too early to tell, but the Ukrainians have been paving the way for an offensive for several weeks now.Michael A. Horowitzanalyst and head of intelligence, Le Beck
It came amid ongoing fears of a nuclear catastrophe at the Russian-occupied Zaporizhzhia nuclear plant, also in Ukraine's south, as an international team of nuclear experts finally set out Monday to inspect the site. Kyiv and Moscow accused each other of continuing to strike the plant despite the risk of a radiation disaster.
NBC News has not verified the claims of either side.
Without a clear picture of what exactly transpired on the battlefield in the south, some military experts were reserved in their analysis.
"It's a bit too early to tell, but the Ukrainians have been paving the way for an offensive for several weeks now," said Michael A. Horowitz, a geopolitical and security analyst and the head of intelligence at Le Beck consultancy.
"I think the Ukrainian military is also keeping a tight lid on tactical information for operational purposes, which to me also means that this is a major offensive," he added.
The apparent offensive could mean the war has now entered its most important stage since Moscow launched its full-scale invasion in February, Melvin said.
"The Ukrainian armed forces are aiming to destroy Russian forces west of the Dnipro River and to reclaim the city of Kherson, and possibly further north to Zaporizhzhia, as well as inflicting heavy losses on Russian forces in occupied areas further east," he added.
"If this can be achieved, it will raise the prospect that Russians can be defeated in the conflict and drive a spike through the heart of Putin's war strategy."