Russia and Ukraine have exchanged prisoners twice since the start of the war, according to Russian Ministry of Foreign Affairs spokesperson Maria Zakharova.

"Two prisoner exchanges have been completed between Russia and Ukraine," Zakharova said in a statement Wednesday.

Russia's ambassador to Poland, Sergey Andreev, said Wednesday that he has received a note from the Polish Ministry of Foreign Affairs demanding the expulsion of 45 Russian diplomats on charges of espionage.

The diplomats must leave Poland within five days, according to Andreev.

The news came after a spokesperson for Poland's Internal Security Agency announced on Polish television that authorities had compiled a list of 45 Russian diplomats in the country who were suspected of spying.

Kremlin spokesman Dmitry Peskov warned Wednesday that Poland's "actions will not be left without retaliation."

Russian shelling hit two residential areas of Kyiv on Tuesday night, according to the city administration.

A shopping center and two private houses were damaged in the Sviatoshynskyi district of the Ukrainian capital, but no one was injured and the fires have been extinguished, officials said.

Several private houses and high-rise buildings were on fire in the Shevchenkivskyi district, where four people were injured. Rescuers and medics were still on the scene Wednesday, and the extent of the damage was under assessment, according to officials.

A 35-hour curfew across Kyiv ended Wednesday morning.

-ABC News' Julia Drozd

Moscow doesn't believe Kyiv's claims of civilian deaths in Ukraine caused by Russian forces, Kremlin spokesman Dmitry Peskov said Wednesday.

"We don't believe the Ukrainian Prosecutor General's Office. Russian troops are carrying out no strikes, aren't firing on civilians," Peskov told reporters during a daily call. "Russian servicemen are helping civilians and, regrettably, more and more eyewitnesses get out of the cities saying that they are being held there as human shields, and that nationalist battalions are firing -- and there are plenty of such cases -- on civilians."

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