WASHINGTON (REUTERS) - Twitter Inc and U.S. authorities are investigating alleged threats made by Islamic State in Iraq and Syria (ISIS) militants against the social media network's co-founder and other employees, according to media reports.

ISIS supporters, in online posts on Sunday, called for attacks against Twitter and its "interests" including death threats, according to the online news media company Buzzfeed, which first reported the story, and NBC.

"Our security team is investigating the veracity of these threats with relevant law enforcement officials," Twitter said in a statement late Sunday, according to reports by CNBC and Buzzfeed.

Representatives for Twitter could not be immediately reached for comment.

One alleged threat was directed to Twitter co-founder Jack Dorsey, NBC reported. Mr Dorsey didn't address or acknowledge the threats in his tweets on Sunday.

ISIS militants have relied heavily on Twitter and other social media technology to coordinate and communicate, including shocking videos of beheadings and other violent acts against its enemies.

But social media companies, including Twitter, have also removed content and suspended accounts that post gruesome content, such as executions.

"Your virtual war on us will cause a real war on you," one online post by ISIS supporters said, according to Buzzfeed. "We told you from the beginning it's not your war, but you didn't get it and kept closing our accounts on Twitter, but we always come back."