UNITED NATIONS (Reuters) - A top Myanmar official appealed on Monday for democracy in the country to be given  a chance to survive  amid international anger over a military campaign against Rohingya Muslims that the United Nations has described as ethnic cleansing. Some 507,000 Rohingya Muslim have fled to Bangladesh since insurgents attacked security posts near the border on Aug. 25, triggering fierce Myanmar military retaliation. Myanmar says more than 500 people have been killed, most of them insurgents. National Security Adviser Thaung Tun told Reuters that he has spoken with his U.S. counterpart, H.R. McMaster, on the phone and visited members of the U.S. Congress in Washington  last week. He said sanctions were not specifically discussed.  The United States called last week on countries to suspend providing weapons to Myanmar s military, but it stopped short of threatening to reimpose U.S. sanctions, which were suspended under the Obama administration.  We need to ensure that democracy has a chance to survive in Myanmar. This is a young democracy,  Thaung Tun told Reuters in an interview at the United Nations, noting that government leader and Nobel peace laureate Aung San Suu Kyi has only been in office for 18 months.  Suu Kyi assumed power following a landslide election win after former military leaders initiated a political transition. For years, the United States and others imposed sanctions on Myanmar in support of Suu Kyi s campaign for democracy.  We have a lot of challenges and these challenges cannot be overcome in a day. Rome wasn t built in a day. So we can t undo 50 years of challenges,  said Thaung Tun, adding that he hoped to meet in person with McMaster, President Donald Trump s national security adviser, to brief him on the current situation. A Trump administration official said last month the violence made it harder for the United States and Myanmar to build warmer ties, and there would likely be some  easing  in the short term, but he did not expect a return to sanctions. Thaung Tun said he had not met privately with U.S. Ambassador to the United Nations Nikki Haley but had spoken to her briefly on the sidelines of other meetings in New York.  The U.N. Security Council met publicly last week on Myanmar for the first time since 2009. Haley said the Myanmar authorities appeared to be carrying out  a brutal, sustained campaign to cleanse the country of an ethnic minority.  She said countries should stop supplying weapons to Myanmar until the military has sufficient accountability measures. Thaung Tun said  action has been taken against a number of policemen who exceeded their bounds in dealing with detainees.   We have very strict instructions for the security forces to carry out their duties within the bounds of the code of conduct,  he said.  Nobody is above the law.  He said that ethnic cleansing or genocide was not happening in Myanmar. The Myanmar government has said that there have no military operations since Sept. 5. But Thaung Tun said that since then security forces have responded when attacked.   Under the cover of night there have been some sporadic attacks and the police and the security people have been required to respond in self-defense,  he said.  The Rohingya are denied citizenship and classified as illegal immigrants, despite claiming roots in Myanmar that go back centuries, with communities marginalized and subjected to bouts of communal violence over the years. Bangladesh and Myanmar agreed on Monday to set up a  working group  to plan the repatriation of those who had fled. (This story has been refiled to correct national security adviser s name on second reference and thereafter to  Thaung Tun , not  Tun .) 