“This is one of the problems because S-400s from the Turkish side put the air defence systems in the area in a different situation", Apostolakis told the audience at the US Institute for Peace in Washington.
Apostolakis explained that Greece will now have to conduct defence plans in a different manner in order to factor in the presence of new weapons systems.
"F-35 [jets] and S-400 completely change the power in the area", Apostolakis said.
In December 2017, Moscow and Ankara signed a loan agreement for the delivery of the S-400 air defence systems to Turkey. The first shipments of the S-400 are expected in June.
The United States and its NATO allies have criticized Turkish military cooperation with Russia, saying the S-400 air defence systems present security concerns given their incompatibility with those in use by NATO.
Turkey is one of the participants in the US F-35 jet program. The United States has threatened Turkey with sanctions for its planned acquisition of S-400 air defence systems and repeatedly said it may delay or cancel sales of F-35 aircraft to Turkey.
Turkish officials have said that purchasing military equipment is a sovereign right and have ruled out the possibility of abandoning plans to acquire the S-400 systems.