Sean Penn Walks to Ukraine-Poland Border After 'Abandoning Our Car' as He Shoots Russia Invasion Doc

Sean Penn is giving an update from Ukraine.

The Oscar winner, 61, traveled to the country last week to continue working on a documentary about Russian tensions, chronicling the invasion and war launched by Russian President Vladimir Putin.

On Monday, he tweeted a photo of himself rolling luggage as he walked along the side of a road, where a traffic jam showed cars bumper to bumper.

"Myself & two colleagues walked miles to the Polish border after abandoning our car on the side of the road. 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," Penn wrote.

Penn said in a statement Saturday, "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. [Ukrainian President Volodymyr Zelenskyy] and the Ukrainian people have risen as historic symbols of courage and principle."

"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," he added.

RELATED GALLERY: Wrenching Scenes from the Start of War as Russia Invades Ukraine

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Russia's attack on Ukraine continues after the country was invaded on Feb. 24, with forces moving from the north, south and east. Details of the attack and the fighting change by the day, but this is the first major land conflict in Europe in decades — and hundreds have already been reported dead or wounded, including children.

Thousands more people have fled or tried to escape Ukraine amid warnings of a possible "refugee crisis."

"You don't know where to go, where to run, who you have to call," Liliya Marynchak, a 45-year-old teacher in Ivano-Frankivsk, Ukraine, told PEOPLE recently of the moment her city was bombed. "This is just panic."

The invasion, ordered by Putin, has drawn widespread condemnation around the world and increasingly severe economic sanctions against Russia. Putin insists Ukraine has historic ties to Russia and he is acting in the interest of so-called "peacekeeping."

Various countries have also pledged aid or military support to Ukraine, whose president, Zelenskyy, has called for peace talks while urging his country to resist.