Ukrainian forces encircled the strategic eastern city of Lyman on Saturday in a counteroffensive that has humiliated the Kremlin, while Russian bombardments intensified after Moscow illegally annexed a swath of Ukrainian territory in a sharp escalation of the war.

In the northeast, Ukrainian officials accused Russian forces of attacking a civilian evacuation convoy, killing 20 people including children.

In the south, Ukraine's nuclear power provider said Saturday that Russian forces blindfolded and detained the head of Europe's largest nuclear plant.

The fighting comes at a pivotal moment in Russian President Vladimir Putin's war.

Facing Ukrainian gains on the battlefield — which he frames as a U.S.-orchestrated effort to destroy Russia — Putin this week heightened his threats of nuclear force and used his most aggressive, anti-Western rhetoric to date.

Despite Purtin's land-grab Friday of four regions in Ukraine, Ukrainian President Volodymyr Zelenskyy and his military have vowed to keep on fighting to liberate the annexed regions and other Russian-occupied areas.

Ukrainian officials said Saturday their forces had surrounded some 5,000 Russian forces who were trying to hold the eastern city of Lyman, which is located in Luhansk, one of the four annexed areas.

Andriy Yermak, Zelenskyy's chief of staff, posted video online Saturday purporting to show Ukrainian soldiers at a monument on the outskirts of Lyman, waving a signed Ukrainian flag.

It remained unclear whether Ukrainian forces have entered the city itself. Luhansk Gov. Serhiy Haidai claimed that all routes to resupply Russian forces in Lyman were blocked.

Russia has not confirmed that its forces were cut off, and Russian analysts had said Moscow was sending more troops to the area.

But the Institute for the Study of War, a Washington-based think tank, said Ukraine likely will retake Lyman in the coming days.

Citing Russian reports, the institute said it appeared Russian forces were retreating from Lyman, 160 kilometers southeast of Kharkiv, Ukraine's second-largest city.

That corresponds to online videos purportedly showing some Russian forces falling back.

Meanwhile Ukrainian authorities accuse Russian forces of targeting two humanitarian convoys in recent days, killing dozens of civilians.

On Saturday the governor of the Kharkiv region, Oleh Syniehubov, said 20 civilians were killed in an attack on a convoy of people trying to flee the Kupiansky district, calling it “cruelty that can't be justified.”

The Security Service of Ukraine, the secret police force known by the acronym SBU, posted photographs of the attacked convoy.

At least one truck appeared to have been blown up, with burned corpses in what remained of its truck bed. Another vehicle at the front of the convoy also had been ablaze. Bodies lay on the side of the road or still inside their vehicles, which appeared pockmarked with bullet holes.

The SBU said the convoy was attacked with “small arms fire,” while the governor said it was shelled. The discrepancy could not be immediately resolved. The exact date of the attack was not announced.

Russian forces have not acknowledged or commented on the attack. Russian troops have retreated from much of the Kharkiv region after a successful Ukrainian counteroffensive last month but have continued to shell the area.

In an apparent attempt to secure Moscow's hold on the newly annexed territory, Russian forces seized the director-general of the Zaporizhzhia Nuclear Power Plant, Ihor Murashov, around 4 p.M. Friday, according to the Ukrainian state nuclear company Energoatom. That was just hours after Putin signed treaties to absorb Moscow-controlled Ukrainian territory into Russia, including the area around the nuclear plant.

Energoatom said Russian troops stopped Murashov's car, blindfolded him and then took him to an undisclosed location.

Russia did not publicly comment on the report. The International Atomic Energy Agency said Saturday that Russia told it that “the director-general of the Zaporizhzhia nuclear power plant was temporarily detained to answer questions.” The Vienna-based agency did not immediately elaborate.