"We are definitively at war with ISIS," Kerry said during an interview on MSNBC's "Morning Joe."
"We are making real progress," Kerry continued. "I think people want to be careful not to herald something in a difficult type of war."
Obama administration officials have long referred to the international effort in more general terms such as "fight" while those covering the efforts have used "war."
Kerry has previously rejected the term "war," insisting in 2014 that the U.S. was engaged in "a major counterterrorism operation."
The administration has eyed a more aggresive position recently as it looks to see ISIS lose ground in the key strongholds of Mosul, Iraq, and Raqqa, Syria.