An ammonia leak has been reported at a chemical plant in the northeastern city of Sumy, the regional governor Dmytro Zhyvytskyy said Monday.

On his official Telegram channel, Zhyvytskyy said the leak was caused by Russian shelling.

He warned those within a 3-mile radius of the Sumykhimprom plant should leave the area because the gas is hazardous but that workers have contained the leak.

Zhyvytskyy said, so far, just one injury has been reported among employees of the plant.

-ABC News’ Joseph Simonetti

More than 3.4 million people have fled Ukraine since Russia invaded, according to the latest figures from the United Nations High Commissioner for Refugees.

Monday's update showed that, of those refugees, more than 2 million have crossed the border into Poland. Additionally, about 535,000 have entered Romania and 365,000 have crossed into Moldova.

Refugees are also going to Hungary, Slovakia, Russia and Belarus.

UNHCR chief Filippo Grandi tweeted Sunday that since the Russian invasion, 10 million people in Ukraine fled, either displaced in the country or as refugees abroad.

"Among the responsibilities of those who wage war, everywhere in the world, is the suffering inflicted on civilians who are forced to flee their homes," he wrote.

UNICEF told ABC News that half of the internally displaced Ukrainians and half of those who have fled are children.

-ABC News' Zoe Magee

Ukrainian officials rejected Russia’s demand that they surrender the southern port city of Mariupol on Monday morning.

“There can be no talk of any surrender, laying down of arms," Deputy Prime Minister Irina Vereshchuk said, according to The Associated Press.

Officials instead called on Russia to allow residents to evacuate safely from the city.

President Volodymyr Zelenskyy in an early morning video address said his government was preparing to send buses to Mariupol on Monday to continue the evacuation.

“In besieged Mariupol, Russian aircraft dropped a bomb on an art school. People were hiding there. Hiding from shelling, from bombing,” Zelenskyy said, according to an official translation from his office. “There were no military positions. There were about four hundred civilians. Mostly women and children, the elderly. They are under the debris. We do not know how many are alive at the moment.”

Some who’ve left Mariupol have described dire circumstances, with constant shelling and little access to essentials, including food, water, and medicine, according to a report published Monday by Human Rights Watch.

“Mariupol residents have described a freezing hellscape riddled with dead bodies and destroyed buildings,” Belkis Wille, senior crisis and conflict researcher at Human Rights Watch, said in a statement. “And these are the lucky ones who were able to escape, leaving behind thousands who are cut off from the world in the besieged city.”

In addition to his trip to Brussels, President Joe Biden will also travel to Warsaw, Poland, on Friday, where he will hold a bilateral meeting with President Andrzej Duda.

"The President will discuss how the United States, alongside our Allies and partners, is responding to the humanitarian and human rights crisis that Russia’s unjustified and unprovoked war on Ukraine has created," White House Press Secretary Jen Psaki said in a statement Sunday.

Psaki's statement did not specify if Biden will do anything else during his trip to Poland, which has taken in more than 2 million Ukrainian refugees since the start of the conflict with Russia.

-ABC News' Molly Nagle

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