An advisor to Ukrainian President Volodymyr Zelenskyy said that U.S. Secretary of State Antony Blinken and U.S. Defense Chief Lloyd Austin are meeting with Ukraine's leader on Sunday in Kyiv.

The adviser, Oleksii Arestovich, said in an interview on Ukrainian TV late Sunday that the talks are going on "right now."

-ABC News' Jason Volack

More than 2.9 million people have fled Ukraine and sought refuge in Poland since the Russian invasion began in February, the Polish Border Guard said on Sunday.

In recent days, however, the number of people crossing the border into Poland has fallen, while the number of refugees going back into Ukraine has risen, according to the border guard.

On Saturday, about 21,100 people entered Ukraine from Poland, while 15,100 fled to Poland from Ukraine, the agency said on Twitter.

Ukraine's losses, including destroyed homes and massive damage to the country's infrastructure, has topped $1 trillion since Russian forces invaded the country two months ago, according to Ukraine's top economic advisor.

Oleg Ustenko, economic adviser the Ukrainian president, announced the country's staggering losses in a televised address on Sunday.

"This is what was destroyed by the Russian invaders in our assets. It is clear that this figure literally increased every day ... the aggressor destroyed our infrastructure, destroyed the homes of our citizens, destroyed everything that could be destroyed by their own and air strikes, and simply by dropping bombs and everything they did," Ustenko said.

Ustenko said additional losses have come as a result of Ukraine's declining gross domestic product.

"It is this figure that we must focus on," Ustenko said. "It is this figure that should be compensated by the Russian Federation immediately after our victory."

The opening of humanitarian corridors for people to escape the besieged Ukrainian port city of Mariupol did not occur as scheduled on Sunday because Russian troops in the area did not confirm a cease-fire, according to Ukrainian Deputy Prime Minister Iryna Vereshchuk.

Vereshchuk called on the United Nations to step in and provide the escape routes for civilians still trapped in Mariupol by the fighting.

She said the problem over establishing humanitarian corridors out of Mariupol should be raised with U.N. Secretary-General Antonio Guterres.

Jalina Porter, the State Department's deputy spokesperson, is responding to the Russian attack at a Ukraine train station that killed at least 50, saying, "We can no longer be surprised by the Kremlin's repugnant disregard for human life."

Five children were among those killed when Russian rockets struck the station in Kramatorsk in Donetsk Oblast on Friday morning, according to Ukraine's state-owned railway company. At least 100 people were injured, according to Donetsk Oblast Gov. Pavlo Kyrylenko.

Russia has denied involvement in the attack, which occurred as "thousands" of civilians fleeing the Russian invasion were at the train station waiting to be taken to "safer regions of Ukraine," according to Kyrylenko.

"Civilians are killed when they stay in their homes, and they're killed when they try to leave," Porter said. "Actions like these demonstrate why Russia did not belong on the U.N. Human Rights Council, and they also reinforce the U.S. assessment that members of Russian forces are committing war crimes in Ukraine."

Porter declined to say if the department considers the train station attack a war crime, saying, "Assessing individual criminal liability in specific cases is the responsibility of courts, as well as other investigatory bodies. But as the secretary, Secretary [of State Antony] Blinken, has said, 'Those responsible for war crimes and other atrocities committed in Ukraine will be held to account.'"

-ABC News' Conor Finnegan