There is an unexploded missile in the immediate vicinity of the nuclear research facility in Kharkiv, Ukraine's regulatory authority told the International Atomic Energy Agency.

The IAEA said it is continuing to monitor the situation.

"The regulator confirmed reports of an unexploded rocket of the multiple launch rocket system 9K58 Smerch 'detected in the immediate vicinity' of a nuclear research facility that has previously been damaged by shelling," the IAEA said in a statement.

It added: "the regulator said 'constant shelling' of the area meant that no measures had yet been taken to dispose of the rocket."

The facility is used for research and development and radioisotope production for medical and industrial applications. The site's nuclear material cannot sustain a fission chain reaction and the radioactive inventory is low, according to the IAEA.

Eight of Ukraine's 15 operational reactors, at four sites, are continuing to operate, including two at the Zaporizhzhya power plant. The other reactors are shut down for regular maintenance, the IAEA said.

-ABC News' Guy Davies

Ukraine's prosecutor's office said that 136 children have been killed in the war and 199 have been injured, as of Saturday morning local time.

The grim update comes on Day 31 of Russia's invasion of Ukraine.

-ABC News' Guy Davies

The Russian army is reluctant to engage in large-scale infantry operations, the British Ministry of Defense said Saturday.

"Russia continues to besiege a number of major Ukrainian cities including Kharkiv, Chernihiv and Mariupol. Russian forces are proving reluctant to engage in large scale urban infantry operations, rather preferring to rely on the indiscriminate use of air and artillery bombardments in an attempt to demoralize defending forces," the Ministry of Defense said in a statement posted online.

It added: "It is likely Russia will continue to use its heavy firepower on urban areas as it looks to limit its own already considerable losses, at the cost of further civilian casualties."

-ABC News' Guy Davies

During a visit to Memphis, Tennessee, on Friday, First Lady Dr. Jill Biden privately met with two Ukrainian families who have loved ones being treated at St. Jude Children’s Research Hospital.

Four Ukrainian children, ages 20 months to 8 years, as well as 10 of their family members, were airlifted from Poland to St. Jude on Monday to continue treatment, according to Michael LaRosa, the First Lady's press secretary.

Biden said in remarks during her visit that her "heart has ached watching the videos" of devastation in Ukraine.

"Parents weeping over their child's broken bones … bodies in the streets. The senselessness of it all is just staggering,” she said.

She added, "When I learned that St. Jude was working with hospitals in Europe to bring some of the Ukrainian children with cancer and their families here, I felt so proud and I wanted to join you in welcoming them. We stand with Ukraine and we're praying for their families."

-ABC News' Armando Tonatiuh Torres-García

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