MOSCOW  - Ex-Soviet Uzbekistan on Thursday pulled out of a Moscow-led regional security alliance after protesting Russian plans to deploy a rapid reaction force for Central Asia near its borders. The decision to leave the Collective Security Treaty Organisation comes as a blow to Russia's efforts to reassert its authority in the resource-rich region in the face a continued diplomatic and economic push from China. "I confirm that the CSTO Secretariat has received a notice of Uzbekistan ending its activities in the organisation," the group's spokesman told the ITAR-TASS news agency without specifying the reason for the decision.Uzbekistan is the biggest and most militarised of Central Asia's four ex-Soviet nations. It is also an important producer of natural gas that supplies Russia's state-owned giant Gazprom.But it never signed on to a 2009 Russian plan to create a rapid reaction force for Central Asia that would ostensibly fight the insurgents and have a base in restless southern Kyrgyzstan.Uzbekistan argued that the move could only stoke regional tensions and provoke new attacks from extremist groups that have also targeted its own government in Tashkent.Uzbek leaders were missing from the most recent CSTO meetings and its membership has been effectively suspended since the rapid reaction dispute emerged.The organisation is sometimes referred to as Russia's answer to NATO because of President Vladimir Putin's hopes of building it up into a regional power that negotiates directly with the Western military bloc.The group includes the other three Central Asian countries and Armenia along with Belarus.