Stoltenberg said on Wednesday that reports of growing Russian military activity were a source of concern, and unhelpful when it came to finding a political solution.
"I am concerned about reports about the increased Russian military presence in Syria," Stoltenberg told reporters during a visit to Prague. "That will not contribute to solving the conflict. I think it is important to now support all efforts to find a political solution to the conflict in Syria."
The comments come amid reports of a growing Russian military presence and build-up of weapons in Syria, with some sources claiming that Moscow has special operations forces fighting in the country to help troops loyal to President Bashar al-Assad.
The Kremlin has been a loyal supporter of Assad's government, supplying it with weapons throughout the four-and-a-half year conflict. However, Russia says its troops are there purely to train Assad's forces how to use Russian equipment.
West creating 'hysteria'
However, Russian Foreign Ministry spokeswoman Maria Zakharova on Wednesday claimed the West was creating "strange hysteria" over Russian activities in Syria.
"Russia has never made a secret of its military-technical cooperation with Syria," she said, adding she could "confirm and repeat once again that Russian military specialists are in Syria to help them master the weapons being supplied."
The Russian Foreign Ministry said Lavrov and Kerry again discussed Syria on Wednesday. The ministry said Lavrov had emphasized the importance of the role of Syrian government troops in confronting extremist groups.
Germany urges restraint
Meanwhile, German Foreign Minister Frank-Walter Steinmeier warned against increased military intervention, claiming that a nuclear agreement with Iran, along with new UN initiatives, opened up new possibilities in finding a political solution. He appeared to be issuing a more general warning, noting that Britain and France were considering air strikes in Syria.
"It can't be the case that important partners, who we need now, back the military option," Steinmeier told German parliamentarians.
French Foreign Minister Laurent Fabius on Wednesday echoed concerns that Russian military activity in Syria might complicate peace efforts. His comments came as France carried out surveillance flights over Syria for a second day, in preparation for launching air strikes.
Britain, whose operations have been mainly limited to Iraq, carried out a solitary drone attack in Syria in August. London claimed that the strike, which killed two British "Islamic State" (IS) fighters, had been an "act of self-defense," with the men plotting a terrorist attack in the UK.
rc/bw (AFP, AP, dpa, Reuters)