Ukrainian forces burst across Russian lines Monday and continued to advance, threatening supply lines for thousands of Russian troops as Moscow contends with devastating military losses on the battlefield. 

Russian sources told Reuters that a Ukrainian tank offensive had advanced dozens of miles along the Dnipro River's west bank while recapturing villages along the way. 

The advance mirrors Ukraine's gains in the east that have turned the tide in Russia's invasion. Moscow recently annexed territory following referendums much of the world refuses to recognize. 

RUSSIAN TROOPS BOXED IN BY UKRAINIAN FORCES AND DNIEPER RIVER, BARGE CARRYING SUPPLIES TO RUSSIAN TROOPS SINKS

In response, Russian President Vladimir Putin announced the mobilization of thousands of troops and threatened nuclear retaliation.  

"The information is tense, let's put it that way, because, yes there were indeed breakthroughs," Vladimir Saldo, the Russian-installed leader in occupied parts of Ukraine's Kherson province, told Russian state television. "There's a settlement called Dudchany, right along the Dnipro River, and right there, in that region, there was a breakthrough. There are settlements that are occupied by Ukrainian forces."

Soldiers from Ukraine's 128th Mountain Assault Brigade raised the country's flag in Myrolyubivka, a village between the former front and the Dnipro, according to a video released by the Defence Ministry.

Serhiy Khlan, a Kherson regional council member, listed Osokorivka, Mykhailivka, Khreschenikvka and Zoloto Balka as villages that had been recaptured from Russian forces. 

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"It means that our armed forces are moving powerfully along the banks of the Dnipro nearer to Beryslav," he said, Reuters reported. "Officially, there is no such information yet, but the (Russian) social media pages which are panicking... absolutely confirm these photos."