NATO Secretary General Jens Stoltenberg urged Russia on Saturday to "take all necessary measures" to ensure NATO airspace was not violated again, after Turkey said a Russian warplane had entered its airspace on Friday despite radar warnings."A Russian combat aircraft violated Turkish airspace yesterday, despite repeated warnings by the Turkish authorities. Previous incidents have shown how dangerous such behavior is," Stoltenberg said in a statement."I call on Russia to act responsibly and to fully respect NATO airspace. Russia must take all necessary measures to ensure that such violations do not happen again."In a similar incident in November, Turkey shot down a Russian warplane flying a sortie over Syria that it said had violated its airspace, triggering a diplomatic rupture in which Russia imposed economic sanctions.