Is this considered an act of war?

Russia has already conducted successful acts of sabotage elsewhere in NATO nations, so apparently not.

https://en.wikipedia.org/wiki/2014_Vrb%C4%9Btice_ammunition_...

https://www.ft.com/content/b1aa5696-6515-487d-ba4d-380c9c431...

https://apnews.com/article/estonia-russia-sabotage-tension-h...

https://www.securityweek.com/german-wind-turbine-firm-disclo...