Penn, 61, tweeted that he and two colleagues abandoned their car on the side of the road and are walking miles to the Polish border. Penn, wearing a backpack and wheeling one suitcase, shared a photo that shows a long line of cars.

"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 tweeted on Monday.

Penn was photographed in Kyiv last Thursday attending a press briefing about the conflict. A source confirmed to Yahoo Entertainment he's making a documentary about the crisis.

"Already a brutal mistake of lives taken and hearts broken, and If he doesn't relent, I believe Mr. Putin will have made a most horrible mistake for all of humankind. President Zelensky and the Ukrainian people have risen as historic symbols of courage and principle," Penn said in a statement. "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."

The untitled project is a Vice Studios production in association with Vice World News and Endeavor Content.

Penn visited the Office of the President of Ukraine during his visit, which praised his efforts in a statement.

"The director specially came to Kyiv to record all the events that are currently happening in Ukraine and to tell the world the truth about Russia's invasion of our country. 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," it read.

"Sean Penn is demonstrating bravery that many others have been lacking, in particular some Western politicians. The more people like that—true friends of Ukraine, who support the fight for freedom — the quicker we can stop this heinous invasion by Russia," the statement added.