MOSCOW, Sept 10 (Reuters) - Russia and ex-Soviet ally Belarus on Friday launched the active "hot phase" of a three-month-long military exercise involving 200,000 troops, which has alarmed Ukraine and some NATO nations. The live fire drills are the culmination of the "Zapad-2021" war games, and will run until next Thursday on Russia"s and Belarus"s western flanks including sites close to NATO and the European Union"s borders. Russia"s Ministry of Defence released footage of warships and tanks opening fire and fighter jets taking off. It said the troops had practiced repelling an aerial attack. Separately, it said it tested S-400 air defence systems on Russian-annexed Crimea. Mathieu Boulegue, a research fellow at Britain"s Chatham House think tank, said the first three days of the "hot phase" drills were expected to focus on defending against an attack from the West, with the next four days focused on how troops would strike back with a counter-offensive. Neighbours such as Ukraine and NATO members Poland and Lithuania say such big exercises so close to the frontier risk being provocative. President Vladimir Putin denies the drills are directed against any foreign power and says they aresensible given increased NATO activity near Russia"s borders and those of its allies. The Western alliance, which has accused Russia ofunder-reporting troop movements in the past, has urged Moscow to be transparent and said it will be watching closely. read more The manoeuvres are held every four years, but this year"s drill has been seen as a particular signal of Russia"s support for Belarus and its leader Alexander Lukashenko, who has been ostracised by the West for cracking down on dissent. Russia sees Belarus as a strategically important buffer to its west, and helped to keep Lukashenko in power with loans and political backing while he crushed a popular uprising last year. Putin and Lukashenko held Kremlin talks on Thursday, described as focusing on the push to more closely integrate their economies. They are due to inspect the exercises personally at some point, according to Belarusian state media. Russia"s Defence Ministry said the active phase of the drills would take place at nine sites in Russia and five sites in Belarus. The exercises involve 200,000 personnel, 80 planes and helicopters, 290 tanks and 15 naval ships, it says. Military personnel from Armenia, India, Kazakhstan, Kyrgyzstan and Mongolia are also due to participate. read more