Machine gun fire echoes through the abandoned buildings of Pripyat in the Chernobyl Exclusion Zone, as Ukrainian National Guard troops stage urban combat exercises. The live-fire training -- carried out in one of the most radioactive places on earth -- comes as warnings swirl over a potential Russian invasion. Moscow has massed over 100,000 troops along Ukraine's border -- including deploying personnel to Belarus, which lies just 10 kilometres (six miles) to the north for joint drills. For Ukraine's forces, the deserted streets and apartment blocks of Pripyat -- empty since residents were evacuated following the nuclear reactor disaster in 1986 -- made an ideal training ground. IMAGES AND SOUNDBITES