Ukrainian Defense Minister Oleksiy Reznikov on Thursday lambasted Russia's military as "a gang of terrorists, criminals and cowards" who she claimed are committing war crimes.

In a statement posted on her official Facebook account, Reznikov marked one month since Russian forces invaded Ukraine and warned that Ukrainians "still have a very difficult period ahead."

"The Russian military machine will not stop until it is drenched in the blood of its soldiers," Reznikov said.

Earlier this month, Russian troops opened fire on a nursing home in the eastern Ukrainian city of Kreminna, killing 56 people, according to Reznikov.

"This is not an accidental hit," she said. "This is the deliberate killing of defenseless people -- a war crime. That's why the Russian army is a gang of terrorists, criminals and cowards."

Still, Reznikov remained confident that Ukrainian forces will prevail with international support.

"We will drive them out. We will rebuild everything," she added. "We will clean our land from the effects of war. It will take a lot of effort and time."

-ABC News' Julia Drozd

Top Russian defense and military leaders have repeatedly declined telephone calls from their U.S. counterparts since Russia invaded Ukraine on Feb. 24, according to Pentagon spokesman John Kirby.

Kirby said in a statement Wednesday that, over the last month, U.S. Secretary of Defense Lloyd Austin and Gen. Mark Milley, chairman of the U.S. Joint Chiefs of Staff, "have sought, and continued to seek, calls with" Russian Defense Minister Sergei Shoigu and Gen. Valery Gerasimov, chief of the General Staff of the Russian Armed Forces. But the Russians "have so far declined to engage," he said.

"We continue to believe that engagement between U.S. and Russian defense leaders is critically important at this time," Kirby added.

-ABC News' Matt Seyler

The United States is slamming Russia's plans to reopen its stock market for limited trading on Thursday for the first time in a month since Russian forces invaded Ukraine.

"What we’re seeing is a charade: a Potemkin market opening," White House deputy national security adviser Daleep Singh said in a statement early Thursday. "After keeping its markets closed for nearly a month, Russia announced it will only allow 15% of listed shares to trade, foreigners are prohibited from selling their shares, and short selling in general has been banned. Meanwhile, Russia has made clear they are going to pour government resources into artificially propping up the shares of companies that are trading."

"This is not a real market and not a sustainable model—which only underscores Russia’s isolation from the global financial system," he added. "The United States and our allies and partners will continue taking action to further isolate Russia from the international economic order as long it continues its brutal war against Ukraine."

Shares plunged and the Moscow Exchange was shut down following Russia's invasion of Ukraine on Feb. 24. The heavy restrictions on trading as the exchange opens Thursday morning aim to prevent the kind of massive selloff that took place last moth in anticipation of crushing financial and economic sanctions from Western nations, according to The Associated Press.

-ABC News' Molly Nagle

A team set up by the White House has been gaming out worst-case scenarios in Ukraine, mostly focused on the possibility Russia carries out chemical and biological attacks, according to a National Security Council official.

The so-called "Tiger Team," set up at the request of national security adviser Jake Sullivan in late February after Russia's invasion of Ukraine began, has analyzed the possibility of Russia using nuclear weapons, but the NSC official emphasized that is not the team's focus. The official said the group is mostly focused on protecting supply chains, security operations of U.S. personnel and planning for chemical or biological weapon attacks.

U.S. officials have repeatedly warned that Russia may be considering using chemical weapons in Ukraine and say Russian allegations that Ukrainians were developing chemical weapons may be a pretense to use such weapons themselves.

-ABC News' MaryAlice Parks

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