The Russians appear to be prioritizing the Donbas region in eastern Ukraine, a senior U.S. defense official told reporters on Monday.

“We're not exactly sure what's behind this reprioritization," the official said. This could be Russia refocusing its strategic goals or trying to gain leverage for talks, the official said.

Meanwhile, in the hard-hit city of Mariupol in southern Ukraine, "the Ukrainians are slugging it out" and "keeping the Russians at bay," the official said.

In Kyiv, the situation is static, the official said. Russian troops have stopped making advances toward the capital city, though they continue using their long-range missile fires, the official said.

"We continue to see Ukrainians defend the city and try to push Russians back," the official said.

-ABC News' Luis Martinez

Germany's economy and climate action minister Robert Habeck says G-7 leaders find Russia's demand for "unfriendly" countries to pay for gas in Russia's currency, rubles, "not acceptable."

"All G-7 ministers agreed completely that this would be a one-sided and clear breach of existing contracts," Habeck said Monday, according to The Associated Press.

"Payment in ruble is not acceptable and we will urge the companies affected not to follow Putin's demand," Habeck said, according to the AP.

Habeck said the leaders of the G-7 nations -- Germany, the U.S., France, Italy, Japan, the United Kingdom and Canada -- discussed the issue on Friday.

Radiation levels remain stable in Russia despite fires in the occupied Chernobyl Exclusion Zone, Russian public health watchdog Rospotrebnadzor said Monday.

Rospotrebnadzor said it was continuing to monitor the situation.

The Chernobyl Exclusion Zone is a 1,000-square-mile restricted area of deserted, contaminated land around the shuttered Chernobyl nuclear power plant, the site of the world's worst nuclear disaster in 1986. Russian forces seized the defunct plant and surrounding exclusion zone just hours after launching an invasion of Ukraine on Feb. 24.

The State Agency of Ukraine on Exclusion Zone Management has warned that the radiation hazard is growing due to the blazes in the area, which it said have the potential to spread. The fires observed at more than 30 spots in the Chernobyl Exclusion Zone over the past two weeks have exceeded 8,700 hectares in total, according to the agency.

However, Vadym Denysenko, an adviser to Ukraine's interior minister, said on Monday that the situation was currently "more or less stable."

Russia is concerned by U.S. President Joe Biden's recent remark seemingly regarding the need for a change of administration in Moscow, Kremlin spokesman Dmitry Peskov said Monday.

In an address on Saturday from Poland's capital, Warsaw, Biden made a comment that appeared to be directed at Russian President Vladimir Putin and his invasion of Ukraine.

"For god's sake, this man cannot remain in power," Biden said.

After the speech, the White House released a statement clarifying that Biden wasn't calling for a regime change.

"The President's point was that Putin cannot be allowed to exercise power over his neighbors or the region. He was not discussing Putin’s power in Russia, or regime change," a White House official said.

When asked by reporters on Monday about Biden's remark, Peskov replied: "Indeed, this statement makes us worry."

"We will continue to closely monitor statements made by the U.S. president," he added. "We are thoroughly recording them and will be continuing to do so."

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