(NewsNation Now) — After visiting Ukraine to film scenes for his documentary on Russia’s invasion, actor and director Sean Penn walked miles to the border to get to Poland.

“Myself & two colleagues walked miles to the Polish border after abandoning our car on the side of the road,” Penn said on Twitter Monday. In the picture he posted to Twitter, Penn is wearing a backpack, and carrying one bag of luggage, with a long line of cars next to him.

“Almost all the cars in this photo carry women & children only, most without any sign of luggage, and a car their only possession of value,” he wrote.

The U.N. refugee agency said Thursday that 1 million people have now fled Ukraine since Russia invaded the smaller country less than a week ago, traveling to neighboring countries by car, train or foot. With an estimated 4 million people leaving Ukraine, the U.N. says this could be the biggest refugee crisis Europe sees this century.

Penn, 61, did not say why he was forced to abandon his vehicle. A spokesperson in Los Angeles, Mara Buxbaum, told Reuters by email on Tuesday that Penn had “made it out of Ukraine safely,” but declined to answer other questions.

Last Thursday, Penn was in Kyiv, attending a press briefing at Ukraine President Volodymyr Zelenskky’s office on the first day of Russia’s invasion, recording footage for the documentary.

“Sean Penn is among those who support Ukraine in Ukraine today. Our country is grateful to him for such a show of courage and honesty,” Zelenskyy’s office said in a statement.

Penn had praise for the Ukrainian people as well, calling them “historic symbols of courage.”

Russia’s attacks on Ukraine are ” a brutal mistake of lives taken and hearts broken,” Penn said.

“Ukraine is the tip of the spear for the democratic embrace of dreams. If we allow it to fight alone, our soul as America is lost,” Penn wrote.

Penn, who has directed movies such as “Into the Wild” and “The Crossing Guard,” is well known for his political activism and involvement in humanitarian causes. He is producing the Ukraine documentary for Vice Studios, a U.S.-Canadian digital media and broadcasting company.