Yes

A cyber attack is a cyber operation, whether offensive or defensive, that is reasonably expected to cause injury or death to persons or damage or destruction to objects.

Michael Schmitt et al; Tallinn Manual 1.0, pp 92
  • Cyber Operation?
  • Expected to cause death or injury to people or objects?
GCHQ Operations to Support Lethal Fires Against ISIS matches this definition. Here's and why this matters.

What happened?

The director of the UK’s Government Communications Headquarters (GCHQ) announced that the organization had been conducting offensive cyber operations against ISIS to suppress their propaganda, disrupt their coordination, and protect deployed military personnel