Complex sentence: Mardakert (also known as) is a province of the "de facto" independent Nagorno-Karabakh Republic, while "de jure" part of the Tartar and Kelbajar rayons of Azerbaijan.
Split into simpler sentences: Mardakert (also known as) is a district of the disputed region of Nagorno-Karabakh. It's a province of the de facto independent Nagorno-Karabakh Republic, and is de jure part of Tartar and Kelbajar rayons of Azerbaijan.