Nato says Russia’s violation of Turkish airspace over the weekend “does not look like an accident”.
Secretary-General Jens Stoltenberg also said Russia had not provided “any real explanation” for two violations, which he called “unacceptable”.
Russia has said the first violation lasted just a few seconds and was due to poor weather. It says it is looking into claims of a second violation.
Russia launched an air campaign in Syria last Wednesday.
Russia says it is targeting Islamic State and other Islamists, but Nato and allied states have expressed concern that Russia is concentrating its attacks on groups opposed to President Bashar al-Assad.
Mr Stoltenberg said Nato and Russia had military-to-military lines of communication open, but added that they had not been used.
He said there had been “a substantial military build-up” by Russia in Syria, including ground troops and naval capabilities.
Russia’s deployment was of “great concern”, he said, calling on Moscow to avoid further escalating tensions with Nato.