Sanctioned countries include Cuba, Iran, North Korea, Syria and the Crimea region of Ukraine
'Slack has banned accounts with alleged links to countries under US sanctions, despite some users of the workplace chat app claiming they only visited them on holiday.
In a message to affected users, Slack said accounts would be closed "effective immediately," in order to comply with laws and regulations imposed by the US government.
Sanctioned countries and regions include Cuba, Iran, North Korea, Syria and the Crimea region of Ukraine.
"In order to comply with export control and economic sanctions laws and regulations promulgated by the US Department of Commerce and the US Department of Treasury, Slack prohibits unauthorised use of its products and services in certain sanctioned countries," the message to affected users stated.
"We've identified your team/ account as originating from one of those countries and are closing the account effective immediately."
Some users affected by the ban took to social media to complain about not being able to appeal the decision.
"So @SlackHQ decided to send me this email," one user wrote on Twitter. "No way to appeal this decision. No way to prove that I'm not living in Iran and not working with Iranians on Slack. Nope. Just hello we're banning your account."
Another user said in a post on the website Hacker News that his wife's account was suddenly closed without warning, resulting in the loss of years of files and messages.
"She created the account while travelling in Cuba (legally) years ago and hasn't been back to Cuba or any other sanctioned country since," the user wrote.'
Read more: Slack bans people who used app while on holiday in countries US doesn't like