Jens Stoltenberg made the statement at a press conference in Brussels
The warning came after reports that Russian jets had flown over Turkish airspace on two separate occasions at the weekend without prior warning or authorisation.
"This is not an accident, this is a serious violation. The violation lasted for a long time compared to previous violations of airspace we have seen elsewhere in Europe,” he said.
Note that, Russian warplanes involved in a bombing campaign in Syria violated Turkish airspace twice on the 3rd and 4th of October.