A missile hit a temporary Russian military camp near the border with Ukraine late Tuesday, according Russian state-owned news agency TASS.

TASS, citing a source, reported that preliminary data shows the camp, just outside the Russian city of Belgorod, was fired on from the Ukrainian side. However, Ukraine has denied responsibility and instead blamed the incident on Russian error.

Belgorod Oblast Gov. Vyacheslav Gladkov said in a statement that blasts occurred in the village of Krasny Oktyabr, about 19 miles southwest of Belgorod. He did not cite a cause of the incident, saying he was awaiting a report from the Russian Ministry of Defense.

Video circulating online and verified by ABC News shows an explosion in Krasny Oktyabr on Tuesday night. The cause of the blast was unknown.

Ukrainian Deputy Prime Minister Iryna Vereschuk alleged that "an unauthorized detonation of ammunition" took place at a warehouse of the Russian Armed Forces in Belgorod.

"This is an example of typical for Russians neglect of safety precautions and mass use of dangerous ammunition of the Second World War," Vereschuk said at a press briefing Wednesday.

Belgorod is about 50 miles north of the Ukrainian city of Kharkiv, which Russian forces have shelled heavily in recent weeks.

-ABC News' Fergal Gallagher

Air raid sirens sounded off across almost all of Ukraine overnight and into early Wednesday, hours after Russia said it would scale back its military operations around Kyiv and Chernihiv.

Russian forces bombarded the besieged northern Ukrainian city of Chernihiv as well as Khmelnytsky Oblast in western Ukraine, while several missiles were shot down over the capital, Kyiv, according to Vadim Denisenko, an adviser to the Ukrainian interior minister. The damage and any casualties were still being assessed Wednesday morning. Meanwhile, the Luhansk Oblast has been under heavy shelling for days, Denisenko said.

The General Staff of the Armed Forces of Ukraine confirmed Wednesday that the Russian military continues to withdraw part of its troops from near Kyiv and Chernihiv, and are possibly "regrouping units to concentrate the main efforts in the eastern direction." However, the General Staff said it believes the real goals of the so-called withdrawal are a rotation of individual units, misleading Ukraine's military leadership and creating an erroneous idea about Russia's refusal from the plan to encircle Kyiv.

-ABC News' Julia Drozd

More than 4 million people have been forced to flee Ukraine since Russian forces invaded on Feb. 24, according to the latest figures from the United Nations Refugee Agency.

The tally from the U.N. High Commissioner for Refugees (UNHCR) amounts to just over 9% of Ukraine's population -- which the World Bank counted at 44 million at the end of 2020 -- on the move across borders in 35 days.

More than half of the refugees crossed into neighboring Poland, UNHCR figures show.

-ABC News' Zoe Magee

The United States is warning that Russian authorities "may single out and detain U.S. citizens" in both Russia and Ukraine.

The warning came Tuesday as the U.S. Department of State issued new travel advisories for the two warring countries.

The State Department previously warned Americans in Russia that they could be targets for harassment by Russian authorities. But the latest advisory makes it explicit that U.S. citizens could be "singled out," "including for detention."

The State Department has also previously warned Americans against traveling to Ukraine to join the fight against Russian forces, pointing to statements from Russian authorities that anyone detained while fighting will not be considered a lawful combatant. That could mean mistreatment or worse, according to State Department spokesperson Ned Price.

"There are continued reports of U.S. citizens being singled out and detained by the Russian military in Ukraine and when evacuating by land through Russia-occupied territory or to Russia or Belarus," the latest advisory for Ukraine states.

Both Russia and Ukraine have been on the State Department's "Travel Advisory Level 4 - Do Not Travel" for months, as tensions ratcheted up and with little to no diplomatic presences on the ground.

-ABC News' Conor Finnegan

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