MOSCOW (Reuters) - Russia said on Monday it was reinforcing its military base in Tajikistan with 30 new tanks and that its forces at the facility had carried out exercises with missile defence systems, highlighting Moscow's concern about turmoil in Afghanistan.

       The possibility of Islamist militants in Afghanistan infiltrating Tajikistan and other former Soviet republics in Central Asia is a worry for Moscow, which views the region as a defensive buffer to its south.

       Russian forces on Monday practiced repelling an enemy aerial attack on military facilities using its S-300PS systems, the Interfax news agency cited the Defence Ministry as saying.

       The base in ex-Soviet Tajikistan, Russia's largest outside its own borders, simulated 10 missile launches against aerial targets as part of the drills, the ministry said.

       "A batch of 30 upgraded T-72B3M tanks with improved combat characteristics will arrive to rearm the 201st Russian military base stationed in Tajikistan on Dec. 6," it was quoted as saying.

       Russia held large-scale joint military exercises with its allies near the Tajik-Afghan border in October after the United States and its allies withdrew troops from Afghanistan and the Taliban swept to power.

       (Reporting by Tom Balmforth; editing by Philippa Fletcher)