The owners of the hotel, the Radisson Blu in the capital Bamako, said 140 guests and 30 employees are "locked in", Sky News reported.
"The Rezidor Hotel Group... is aware of the hostage-taking that is ongoing at the property today, 20th November 2015," the company said in a statement.
"As per our information two persons have locked in 140 guests and 30 employees."
A member of staff who gave his name as Tamba Diarra said over the phone the attackers used grenades in the assault.
Reports of gunfire surfaced on Friday morning on social media, although a defense ministry adviser said it was not clear what had happened.
Following a military coup in 2012, extremists took control of northern Mali.
The extremists were scattered from northern towns and cities, but the north remains insecure and militant attacks have extended farther south this year.
In March masked gunmen shot up a Bamako restaurant popular with foreigners, killing five people.