Photos capture heartbreak of separated families in Ukraine

As hundreds of thousands of Ukrainians scramble to leave the country in the wake of attacks from Russia, images of children torn from their families and their homeland are capturing the world's attention.

In one image, captured by ABC News' Matt Gutman, a sobbing young girl clutches a stuffed animal as she and her mother leave on a train from the railway station in Lviv, in west Ukraine.

The girl and her mother were separated from her father, forcing the mother to make a decision to get on the train to Poland without him, according to Gutman.

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"The girl kept crying, clutching her stuffed animal. Her mother also sobbing, until the train pulled out from the station," he wrote on Instagram. "They didn’t get to say goodbye."

Over the past six days, over 670,000 refugees have fled Ukraine to neighboring countries, according to the United Nations High Commissioner for Refugees (UNHCR).

In Poland, where the girl and her mom were headed, most arrivals are women and children, many of whom waited up to 60 hours to board a train out of Ukraine, according to the UNHCR.

Last week, as Russia began to launch military strikes, the government in Ukraine enacted martial law, requiring men to stay in the country.

A spokesperson for UNICEF, the United Nations Children's Fund, said Tuesday there are "anecdotal reports of heartbreaking stories of fathers -- or families -- arriving at the border with their children and relinquishing them to the border guards, then remaining in Ukraine.”

Filippo Grandi, the head of the UNHCR, also confirmed the refugees include a "growing number of unaccompanied and separated children."

"We are helping and can do more in areas like protection and registration, organizing reception capacity, providing emergency relief and cash assistance, and in identifying and responding to the needs of the most vulnerable, many of them women and children, including a growing number of unaccompanied and separated children," Grandi said Monday night in remarks to the U.N. Security Council. "But we know that we are not even scratching the surface to meet the needs of Ukrainians, including an unknown but surely very substantial number who have been forced to flee their homes over the past days."

Relief organizations have stepped up to helprefugees fleeing Ukraine. Some, like the International Rescue Committee, are focusing specifically on families.

“We’re extremely concerned about the rising humanitarian needs in the country. Thousands of people who have fled their homes are currently without basic necessities including shelter and food,” IRC’s senior director of emergencies, Lani Fortier, said in a statement on the charity's website. “In displacement contexts, women and girls are always the most adversely affected and bear the brunt of crises. The situation in Ukraine is no different. Women and girls, especially those traveling alone could be at risk of exploitation and abuse.”

ABC News' Zoe Magee and Robert Zepeda contributed to this report.