"We have seen encouraging signs that the ceasefire is largely holding, but at the same time we have seen some reports about violations," Stoltenberg told a press conference in Kuwait City.
"Of course, that is of concern because it is important that all sides should respect the agreement," which is the best way to renew efforts for a political solution to the devastating five-year conflict, he said.
The ceasefire deal was brokered by Moscow and Washington and took effect at midnight on Friday. It entered its third day largely intact today despite accusations of violations from both sides.
"We are concerned about the Russian military buildup we have seen in Syria," whether it is military or air force, he said.
The NATO chief said Russian air strikes had "mainly targeted" non-jihadist rebels rather than the Islamic State group, which has been targeted by a US-led coalition.
He said the Western alliance had no plans to send ground troops to Syria as part of the campaign against the jihadists.