On Monday, the Kremlin expressed concern over remarks made by US President Joe Biden, wherein he called Russian President Vladimir Putin a "butcher" over the war in Ukraine.

"This is a statement that is certainly alarming," Kremlin Spokesperson Dmitry Peskov told reporters, adding that they will continue to closely monitor statements made by Biden.

During a visit to Poland on Saturday, Biden said his Russian counterpart "cannot remain in power," calling the conflict in Ukraine Moscow's "strategic failure". 

Peskov had earlier addressed Biden's remarks on the day he made them, saying it is weird to hear US President Joe Biden, the man who called for bombing Yugoslavia and killing people, raising accusations against Russia's President. He added that Biden's personal insults against Putin narrow the window of opportunity for improving relations between Moscow and Washington.

"Still, a state leader should keep their temper," he told TASS.

Russian POW shooting probe, Russia-Ukraine Talks

Peskov also addressed talks with Ukraine, saying that no major breakthroughs have been made so far, with a new round of talks set to take place in Istanbul.

"So far we cannot state any significant achievements or breakthroughs," he said.

"For now we cannot and will not speak of progress," adding that it was important that it's been decided to continue the talks in person.

As for the shooting of Russian prisoners, Kremlin vowed to hold those responsible accountable.

Russia has begun an investigation after a violent video purportedly showing the abuse of prisoners of war at the hands of the Ukrainian military surfaced online.

The Russian investigative committee stated Sunday that “the video circulating online shows captured soldiers being shot in the legs with medical assistance."

Alexander Bastrykin, the head of the committee, instructed investigators to "establish all the circumstances of the incident, collect and record evidence and identify all the persons involved in it to subsequently bring them to justice.”

Last week, the head of the "Mobile Hospital" project in Ukraine's army, Gennady Druzenko, called on doctors under his command to castrate Russian prisoners during an interview with channel Ukraine 24.

He said he gave strict orders for them "to castrate all the wounded men because they are cockroaches, not humans."