At least 139 children have been killed since the Russian invasion of Ukraine began on Feb. 24, the Ukrainian Prosecutor General’s Office said.

Another 205 children have been injured, the office said on Sunday.

Russian forces in northern Ukraine have been “largely static,” as Ukrainian resistance and counterattacks have been “hampering” their attempts to reorganize, the UK Ministry of Defence said on Sunday.

“Russian forces appear to be concentrating their effort to attempt the encirclement of Ukrainian forces directly facing the separatist regions in the east of the country, advancing from the direction of Kharkiv in the north and Mariupol in the south,” the Ministry’s update said.

Ukrainian President Volodymyr Zelenskyy reiterated his call for 1% of NATO's arsenal during his latest national address.

"One percent! We did not ask for more. And we do not ask for more. And we have already been waiting for 31 days!" he said.

Zelenskyy, who similarly pleaded for more military aid during an address at this week's NATO summit, said Ukraine is especially in need of more planes and tanks.

"Ukraine cannot shoot down Russian missiles using shotguns, machine guns, which are too much in supplies," he said. "And it is impossible to unblock Mariupol without a sufficient number of tanks, other armored vehicles and, of course, aircraft."

Zelenskyy said Ukraine's minister of foreign affairs and the minister of defense met with U.S. officials, including President Joe Biden, in Poland Saturday to address these "vital interests."

President Joe Biden spoke on the phone with Belarusian opposition leader Svyatlana Tsikhanouskaya from Air Force One, the White House said Saturday.

Biden "underscored the continued support of the United States for the Belarusian people in defending and advancing human rights, including freedom of expression, and free and fair elections," the readout of the call stated.

Tsikhanouskaya attended Biden's address in Warsaw on Saturday, according to the readout.

Belarus has served as a staging area for Russian attacks on Ukraine during the war. Its president, Alexander Lukashenko, is a close ally of Russian President Vladimir Putin.

-ABC News' Armando Garcia

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