As Russia's attempts to capture Ukrainian territory have been slowed by Ukrainian resistance, the invading forces have switched to a “strategy of attrition,” the UK Ministry of Defence said on Saturday.

"This is likely to involve the indiscriminate use of firepower resulting in increase civilian casualties, destruction of Ukrainian infrastructure, and intensify the humanitarian crisis," the Ministry said on Twitter.

Ukrainian President Volodymyr Zelenskyy reacted to the massive concert that occurred Friday in Moscow in support of Russia’s invasion of Ukraine and on the eight-year anniversary of Russia’s annexation of Crimea from Ukraine.

"A big rally took place. And I want to pay attention to one detail. It is reported that a total of about 200,000 people were involved in the rally in the Russian capital -- 100,000 on the streets, about 95,000 at the stadium. Approximately the same number of Russian troops were involved in the invasion of Ukraine," Zelenskyy said in his latest national address Friday night.

"Just imagine 14,000 corpses and tens of thousands of wounded and maimed people at that stadium in Moscow," he continued. "There are already so many Russian losses as a result of this invasion. This is the price of war. In a little more than three weeks. The war must end."

Zelenskyy noted progress in evacuating more than 180,000 Ukrainians through humanitarian corridors, though charged that Russian invaders are blocking the supply of humanitarian aid to some besieged cities.

"This is a totally deliberate tactic. They have a clear order to do absolutely everything to make the humanitarian catastrophe in Ukrainian cities an 'argument' for Ukrainians to cooperate with the occupiers," he said. "This is a war crime. They will be held accountable for this. 100%."

President Joe Biden held a video call with Chinese President Xi Jinping for one hour and 50 minutes on Friday, marking the first time the two leaders spoke since November.

The White House readout of the call doesn't say whether the conversation was constructive or not, but the White House said Biden made clear the "implications and consequences" if China aligns with Russia and provides them "material support."

China's readout of the call said China supports negotiations but passes the buck to the U.S. and NATO to "conduct dialogue with Russia to solve the crux of the Ukraine crisis and resolve the security concerns of both Russia and Ukraine."

The call was “direct,” “substantive” and “detailed,” according to a senior administration official.

The official said Biden “really wasn't making specific requests of China” on the call and instead was “laying out his assessment of the situation, what he thinks makes sense, and the implications of certain actions.”

The official said that the call was “less about coming away with a particular view out of conversation today and more about making sure, again, that they were able to really have that direct candidate and detailed and very substantive conversation at the leader level.”

-ABC News' Mary Bruce, Karson Yiu and Justin Gomez

French President Emmanuel Macron spoke with Russian President Vladimir Putin on the phone Friday, sharing "his extreme concern" about Russian attacks on the besieged city of Mariupol, the Élysée said.

Macron "asked him for concrete and verifiable measures to lift the siege of Mariupol, humanitarian access and an immediate ceasefire," the Élysée said.

Russian attacks have prevented many civilians from escaping Mariupol and is keeping humanitarian supplies from being brought in. The Mariupol City Council reported Sunday that 2,187 residents had been killed since the start of the invasion. Ukrainian Deputy Prime Minister Iryna Vereschuk said last week that the city was "beyond a humanitarian disaster," with most roads destroyed, little communication with the outside and no power, gas or heat.

All Russian troops have left the Ukrainian cities of Kyiv and Chernihiv, withdrawing north toward the borders of Belarus and Russia to consolidate before likely redeploying to the Donbas region in eastern Ukraine, a senior U.S. defense official told reporters Wednesday.

But even with the Russians gone, the territory remains treacherous.

"There are some indications that they left behind mines and things like that, so the Ukrainians are being somewhat careful in some areas north of Kyiv as they begin to clear the ground and clear the territory and re-occupy it," the official said.

While the U.S. hasn't yet seen these troops redeploy elsewhere in Ukraine, it'll likely happen soon, according to the official. Ukrainian forces are preparing for a major fight in Donbas, the official said.

The official also said the Pentagon is "monitoring" an apparent nitric acid explosion in Ukraine's Luhansk region, which Russia blamed on Ukraine.

"We've seen the Russians claim that this was a Ukrainian attack on this. We do not believe that is true," the official said. "We do believe that the Russians are responsible, but exactly what they used when they did it, why they did it, what the damage is, we just don't have that level of detail," the official said.

The official also noted that a small number of Ukrainians currently in the U.S. for "professional military education" were pulled aside for a couple days of training on Switchblade drones, which the U.S. is sending overseas as part of its military aid, according to the official.

"Although it's not a very difficult system to operate, we took advantage of having them in the country to give them some rudimentary training on that," the official said.

-ABC News' Matt Seyler