BRUSSELS - NATO Secretary General Jens Stoltenberg said "Russia's deployment of significant forces in Syria is of great concern" at a press conference Tuesday.

Defense ministers of NATO countries will address the situation in Syria, Ukraine and Afghanistan, as well as the reinforcement of collective defense under the growing challenges from east and south on Thursday's meeting.

Stoltenberg said that Russia has built up a substantial military presence including ground troops in Syria over the last weeks, on which NATO has expressed deep concern.

NATO chief stressed that "Russian combat aircraft have violated Turkish airspace," calling on Russia to de-conflict its military activities in Syria.

In response to a question from media, Stoltenberg answered "It doesn't look like an accident and we also have seen two of them during the weekend."

Russian Defense Ministry on Monday blamed bad weather conditions for the brief violation of Turkish airspace by a Russian aircraft near Syrian border.

"The incident was a consequence of unfavorable weather conditions in the area. One should not look for any conspiracy theories here," the ministry said.  (Xinhua)