MOSCOW (Reuters) - Moscow will work more closely with fellow members of a post-Soviet security alliance as the withdraw of U.S. forces from Afghanistan raises regional security risks, Russia's defence minister was quoted as saying on Saturday.

       The U.S. exit from Afghanistan has created a security headache for Moscow, which sees former Soviet Central Asia as part of its southern defensive flank and fears the spread of radical Islam.

       Russian Defence Minister Sergei Shoigu said Moscow had stepped up contacts within the Collective Security Treaty Organization (CSTO), a Moscow-led alliance that groups six former Soviet states.

       "We're in close contact with CSTO members over Afghanistan ... That's why our interaction within CSTO should be strengthened," Shoigu was quoted as saying by the RIA news agency.

       He said the risk of Afghan militants crossing into neighbouring countries had spread following the Taliban's rapid takeover of the country this month as U.S. forces withdrew.

       Drug trafficking is another concern, he added.

       The CSTO said on Friday it would hold military exercises in Kyrgyzstan and Tajikistan from September to October involving several thousand troops due to the situation in Afghanistan.

       Shoigu said an ongoing programme to rearm the armies of Kazakhstan, Kyrgyzstan and Tajikistan would continue.

       (Reporting by Vladimir Soldatkin; Editing by Helen Popper)