LONDON (Reuters) - Myanmar is facing a  defining moment  and must stop the violence against its ethnic minority Rohingya population, U.S. Secretary of State Rex Tillerson said on Thursday. Attacks by Rohingya militants on security posts last month triggered an army operation that has killed more than 400 people, destroyed over 6,800 houses and sent nearly 400,000 Rohingya Muslims fleeing to Bangladesh. On a visit to London where he met British Prime Minister Theresa May and foreign minister, Boris Johnson, he told a news conference:  I think it is a defining moment in many ways for this new, emerging democracy.  He said he understood that Myanmar s Aung San Suu Kyi, a Nobel prize laureate and de facto head of the government in Myanmar, was in a power-sharing agreement with the military and the  complex situation  in which she found herself.  I think it is important that the global community speak out in support of what we all know the expectation is for the treatment of people regardless of their ethnicity,  he added.  This violence must stop, this persecution must stop.  