The creators of the new television drama Confederate, set in an alternate world in which slavery still exists in the US, have defended their vision after the concept provoked outrage on social media.

“This is scary, for all of us … It is a pretty terrifying prospect getting into it. We knew it would be and now it’s come true,” David Benioff, one of the executive producers of Confederate, told pop culture website Vulture on Thursday.

Benioff and Dan Weiss, the creators of the hugely successful medieval fantasy TV series Game of Thrones, announced earlier this week their next TV project would be Confederate – set in an alternative reality where slavery is still legal, southern US states have seceded and the country is on the brink of a third civil war.

The show is still in the early stages of development for premium cable channel HBO but the premise provoked a backlash on social and pop-culture media as offensive and inappropriate coming from two white men.

Daily Beast writer Ira Madisson III wrote that the show “sounds stupid as hell”.

“Give me the confidence of white showrunners telling HBO they wanna write slavery fanfic,” tweeted Pilot Viruet, a journalist with Vice.com.

“Good luck finding black actors for this project,” David Harewood, an actor on Homeland, said on Twitter.

Benioff and Weiss will be working with two black writers – Malcolm Spellman and Nichelle Spellman – and told Vulture they did not yet have an outline or even characters for the show.

Malcolm Spellman acknowledged that the premise was “weapons-grade material”, but he said the series would provide an opportunity to look at how racism and white supremacy are “alive and real” in the US, more than 150 years after slavery was abolished.