NATO leaders reviewed the implementation of the Alliance's Readiness Action Plan on Thursday in response to growing Russian aggression in Syria.

NATO Defense Ministers approved the Enhanced NATO Response Force plan, marking the largest reinforcement of the alliance's defenses since the end of the Cold War. The ministers also agreed to set up NATO Force Integration Units, or small headquarters, in Hungary and Slovakia.

"We now have everything in place to make the Response Force stronger, faster and more capable," said NATO Secretary General Jens Stoltenberg, reassuring member states the alliance is bolstering their defenses, and intends to keep them safe from threats.

Further steps include increasing military exercises. Britain, a NATO member, also announced that it will send more troops to Ukraine and increase training programs for Ukrainian troops fighting Russian-backed separatists in the country.

Russian intervention in Syria, including airstrikes and the launch of cruise missiles from Russian Navy ships in the Caspian Sea, some of which reportedly ended up striking Iran, has been a cause for NATO concern.

Russian aircraft violated Turkish airspace during the course of its operations, and U.S. and Russian aircraft had a near miss as both countries conducted airstrikes over Syria.

Stoltenberg has called the violation of Turkey's airspace deliberate and "unacceptable", advising Moscow authorities to "avoid escalating tensions."

"This sends a clear message to all NATO citizens: NATO will defend you. NATO is on the ground. NATO is ready," Stoltenberg said of Thursday's measures.

NATO officials also discussed security issues in Afghanistan, concluding that troop levels will be determined following a detailed security assessment in the coming weeks.