Kharkiv, Ukraine — The leader of Russia's notorious Wagner mercenary group, which for months led Moscow's grueling effort to capture the eastern Ukrainian city of Bakhmut, on Monday called it a "disgrace" that Ukraine's troops had managed to recapture ground near the town.  

Ukrainian forces mounted attacks on multiple locations along the long front line over the weekend, but officials in Kyiv remain tight-lipped as to when a long-anticipated, large-scale counteroffensive might begin in earnest.

• Blinken says "stronger" Ukraine a prerequisite for talks with Russia

Yuriy Sak, an advisor to Ukraine's military, told CBS News' partner network BBC News on Monday that the "next stages" of the country's effort to repel Russia's invasion would not be formally announced, "but more importantly," he said, they "never stopped." 

Ukraine's fight along the front lines was "never idle," he said, adding that it was "some days more intense, some days less" but stressing that around Bakhmut in particular, over the last couple weeks Ukrainian forces "have been counterattacking."

Sak noted the departure of the Wagner forces from that battle and said it was his side's "understanding" that "Russian losses around Bakhmut have been around seven-and-a-half times larger than ours."

He said it was all part of Ukraine's military strategy to prepare "for the next stage of the offensive... moving little by little" to reclaim all occupied territory from Russia.

• Russia accused of using "starvation tactics" against Ukrainian civilians

Sak urged people not to accept anything Moscow claimed about the state of the war, telling the BBC that Russian officials were "lying on a daily basis about everything," including by trying to pass off "old footage of destroyed military equipment" as new.

President Volodymyr Zelenskyy has said his armed forces are ready, but he's avoided making any predictions as to how, where or exactly when it will unfold.

Russia's Ministry of Defense released video over the weekend showing what it said were its troops repelling a Ukrainian advance in the eastern region of Donetsk, much of which Russian forces have occupied for many months. It's one of the eastern Ukrainian regions that President Vladimir Putin illegally declared annexed by Russia last year, though his forces have never fully controlled the territory.

CBS News cannot verify the Russian video, or the claims made with it, and Ukrainian officials have consistently sent mixed signals about when the counteroffensive will begin. That could be a deliberate military tactic in itself.

A video released over the weekend by Ukraine, with the tagline "Plans Love Silence," shows Ukrainian soldiers urging operational secrecy, putting their fingers to their lips to shush any talk of the much-hyped major counteroffensive.

Across Ukraine's eastern border inside Russia, meanwhile, a new front line has seemingly opened up. Anti-Kremlin militia groups, fighting alongside Ukrainian forces, have stepped up attacks in Russia's Belgorod border region.

Zelenskyy says 500 Ukrainian children killed

With the threat of exploding drones and even ground attacks, thousands of Russians are being forced to take refuge in shelters — now feeling the hardships that Ukrainian civilians have become so accustomed to since Vladimir Putin ordered the full-scale invasion 15 months ago.  
 
"We are trying to be strong," said Irina Burlakova, a Russian mother taking shelter in Belgorod over the weekend, "because we have children who give us the incentive to carry on."

Children were at the center of yet another Russian missile attack, meanwhile, near the Ukrainian city of Dnipro early Sunday morning. Rescuers worked frantically, but emerged from the rubble with the devastating news that at least five children were injured in the strike, which hit an apartment building, and a two-year-old girl was killed as she slept with her mother, who was left fighting for her life in a hospital.  

President Zelenskyy said over the weekend that 500 Ukrainian children had been killed since the war began — children, he said, who "could have become famous scholars, artists, sports champions, contributing to Ukraine's history."

He warned the real number was likely higher, adding: "We must hold out and win this war! All of Ukraine, all our people, all our children must be free from the Russian terror."

CBS News' Tucker Reals contributed to this report.

In:
• War
• Ukraine
• Russia
• Vladimir Putin