"We are experiencing a terrible humanitarian catastrophe in Syria. Sometimes, however, the cost of using the military means is higher than their benefits. Regarding Syria, the NATO members agreed that the use of the armed forces would make the horrible situation even more horrible," Stoltenberg told German Bild newspaper in an interview.
He added that the military intervention may have prompted a bigger escalation of the conflict.
The civil war has been raging in Syria since 2011, with government forces loyal to President Bashar Assad fighting numerous opposition factions and extremist groups. The Islamic State extremist group, outlawed in Russia and many other countries, has been able to capture large territories in both Syria and Iraq since 2014.