US actor and comedian Ben Stiller said Tuesday he had been deeply affected by the stories he heard from Ukrainians after a surprise visit to the war-torn country.

The "Zoolander" and "Meet the Parents" star was in Ukraine as an ambassador for the UN's refugee agency, UNHCR.

"The stories of the people who experienced the first few days of this war -- speaking to them and hearing these really harrowing stories -- was very affecting," Stiller told AFP in a phone call, while travelling from Ukraine to Poland.

The 56-year-old actor-director visited Irpin, a suburb of Kyiv, which was particularly hard-hit in the early stages of the Russian invasion.

"To see the physical destruction of those neighbourhoods, it's really massive, and it's impossible not to be affected by that," Stiller said.

"There was one very young man who... talked about it like something out of a horror film, just being hit by these missiles, not knowing if he should stay or run.

"To see how traumatised he was -- he felt his life had been changed forever. It's really tough when you hear a young person say that, and you realise the effects of war are not always visible."

Ben Stiller told Volodymyr Zelensky: 'You're my hero' during a visit to Kyiv
Ben Stiller told Volodymyr Zelensky: 'You're my hero' during a visit to Kyiv UKRAINIAN PRESIDENTIAL PRESS SERVICE via AFP / Handout

Stiller met with Ukraine President Volodymyr Zelensky in Kyiv on Monday, and told him: "You're my hero."

"What you've done, the way that you've rallied the country, the world, it's really inspiring," he told Zelensky.

Asked by AFP if he could imagine trading in his job as a comedian in the event of an invasion, Stiller said he didn't know how he would react.

"If everything you know is in danger of being obliterated -- who knows? But I hope we could react like the people of Ukraine," he said.

"Yesterday, I was in a house that was almost fully collapsed with a woman, and we were sitting in her kitchen," he said.

"And she was giving us strawberries and had this incredible resilience saying: 'We have to figure out how to go forward'. It's hard not to think 'how would I react if my house had half collapsed.'"

"The work that UNHCR and other NGOs do is so important. This war has created 12 million displaced people and even if the war ends, the issues it has created will go on for years," he said.