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.

Mariupol has not yet fallen, despite Russia's demands that Ukrainian troops defending the besieged Ukrainian port city surrender, according to Ukrainian Prime Minister Denys Shmyhal.

"There [are] still our military forces, our soldiers, so they will fight until the end," Shmyhal told ABC News' George Stephanopoulos in an exclusive interview Sunday on "This Week."

Mariupol is a strategic city for Moscow because it would allow Russian forces in the south to connect with troops in eastern Ukraine's contested Donbas region. It would also give Moscow a key port.

Although Mariupol remains under the Ukrainian government's control, Shmyhal said the city's residents are suffering.

"They have no water, no food, no heat, no electricity," he said. "They ask all of our partners to support and help stop this humanitarian catastrophe."

-ABC News' Monica Dunn