"What is going on in Syria of course is of importance for NATO because we see that Russia is testing some of their most modern weapons," Stoltenberg said on Thursday at a press conference at the headquarters of NATO where defence ministers of the organisation's member states are meeting.
The secretary general added that he is also worried about increased Russian military presence and its relation to the violation of the NATO airspace.
Stoltenberg said that NATO defence ministers have discussed the escalation of the Russian military action in Syria, and agreed that the situation posed "serious concerns".
"I call on Russia to play a constructive role in the fight against IS. But Russian actions and the support to the regime are not helpful," he said.
He also reiterated that the recent violations of Turkish airspace by Russian aircrafts are "unacceptable" and said NATO is monitoring the situation closely.