March 29 (Reuters) – Russia has begun exercises with its Yars intercontinental ballistic missile system and several thousand troops, its defence ministry said on Wednesday, in what is likely to be seen as another attempt by Moscow to show off its nuclear strength. President Vladimir Putin has aimed to make the Yars missile system, which replaced the Topol system, part of Russia's "invincible weapons" and the mainstay of the ground-based component of its nuclear arsenal. "In total, more than 3,000 military personnel and about 300 pieces of equipment are involved in the exercises," the defence ministry said in a statement…