Mark Ruffalo has insisted Ellen DeGeneres shouldn't even consider being friends with former U.S. President George W. Bush until he's "brought to justice for the crimes of the Iraq War".

The chat show host hit headlines on Sunday when she and her wife Portia de Rossi were seen enjoying a Dallas Cowboys game with George and his wife Laura.

After Ellen attracted criticism online for her friendship with Bush, famous for his anti-LGBTQ stances and decision to go to war in Iraq, she hit back and insisted: "I'm friends with a lot of people who don't share the same beliefs that I have. We are all different and I think we've forgotten that that's OK that we're all different."

However, Avengers star Ruffalo, who frequently makes waves with his outspoken political views, targeted Ellen on Wednesday as he shared a Variety article headlined "Ellen DeGeneres, George W. Bush and the Limits of Unconditional Kindness".

Alongside the link, Ruffalo wrote: "Sorry, until George W. Bush is brought to justice for the crimes of the Iraq War, (including American-lead torture, Iraqi deaths & displacement, and the deep scars-emotional & otherwise-inflicted on our military that served his folly), we can't even begin to talk about kindness."

The chat show host did receive support from celebrity pals including Reese Witherspoon and Kristen Bell, who praised Ellen for her statement about the situation.

"People were upset. They thought, 'Why is a gay Hollywood liberal sitting next to a conservative Republican president?'" DeGeneres said on her popular U.S. talk show. "When I say, 'Be kind to one another,' I don't only mean the people that think the same way that you do. I mean be kind to everyone. Doesn't matter."