Kate Green/Getty Images Jessica Chastain
The Oscar-winning actress, 44, took to Instagram Thursday to share footage of her trip to the region in early August, writing in a caption that she "needed a few weeks to process everything" she saw.
The trip began in Poland, she wrote, where her group landed before driving four hours and taking a train for another 11 hours, then arriving in Kyiv.
"I was nervous and the length of the journey gave me time to calm myself," she wrote.
In her video, Chastain can be seen in a car as it drives by buildings reduced to rubble, and visiting with children as she notes that thousands of civilians have been killed in the war — with many more displaced.
The number of refugees from Ukraine "now stands at 5.1 million who've left for the European Union countries, another 7.7 million were internally displaced."
Elsewhere in the footage, the actress writes messages of encouragement for the children, such as, "You are so brave," and accepts handmade artwork a young girl made her as a gift.
The powerful trip included a visit to a children's hospital, she added, writing that she met with children who had been displaced amid the war.
"One of the experiences from this life changing trip was my visit to the children's hospital: Ohmatdyt," Chastain wrote. "I had the opportunity to meet incredible children and the director, Volodymyr Zhovnir."
She continued: "The children amazed me with their hope and strength. Many had been displaced from their homes and spent months in the hospital."
As Chastain noted, the displaced children are illustrative of "a huge humanitarian crisis going on in the Ukraine."
"We can't forget the innocent ones effected by senseless acts of violence," Chastain wrote. "I will always be grateful to have met these sweet little kids with big brave hearts."
While in Ukraine, Chastain met with Ukrainian President Volodymyr Zelenskyy, who Variety reports shared photos of the meeting on Telegram, along with the caption: "American actress Jessica Chastain is in Ukraine today. For us, such visits of famous people are extremely valuable. Thanks to this, the world will hear, know and understand the truth about what is happening in our country even more. Thanks for the support!"
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Meanwhile, Russia's attack on Ukraine continues after its forces first launched a large-scale invasion on Feb. 24.
ANDREY BORODULIN/AFP via Getty The Russian invasion of Ukraine
Putin insists Ukraine has historic ties to Russia and he is acting in the best security interests of his country. Zelenskyy meanwhile, has vowed not to bend.
"Nobody is going to break us, we're strong, we're Ukrainians," he told the European Union in a speech in the early days of the fighting, adding, "Life will win over death. And light will win over darkness."
Russia has denied targeting civilians but Zelenskyy and other world leaders, including U.S. President Joe Biden, have accused the invading forces and Putin of war crimes, including genocide, igniting calls for investigations and, eventually, accountability.
The Russian attack on Ukraine is an evolving story, with information changing quickly. Follow PEOPLE's complete coverage of the war here, including stories from citizens on the ground and ways to help.