DHAKA (Reuters) - The U.S. government will constantly monitor its sanctions regime to respond to violence in Myanmar s Rakhine state that has sent hundreds of thousands of Rohingya Muslim refugees across the border to Bangladesh, a senior U.S. official said on Saturday.  We are constantly evaluating the situation,  Simon Henshaw, Acting Assistant Secretary of State for the Bureau of Population, Refugees, and Migration, told reporters at a news briefing in Dhaka after visiting camps in Cox s Bazar where hundreds of thousands of Rohingya refugees are sheltering.  Congress has given us a number of tools we can use,  he said, noting disturbing reports of atrocities in Rakhine state.   We call for a full investigation of these reports, we call for those who committed (the atrocities) to be held accountable,  he said.  It is incredible that only within two months more than 600,000 refugees have taken shelter in a country.  A seven-member delegation led by Henshaw visited Myanmar from Oct. 29 before traveling to Bangladesh on Wednesday. It went to Cox s Bazar on Thursday.  During our meetings with Myanmar government officials we told them that it is their responsibility to return a secure and stable situation in Rakhine state. It is also their responsibility to investigate the reports of atrocities.   The delegation also asked Myanmar to allow Rohingya people back to their land and to rehabilitate the villages that were burned. He said they had been shocked by Rohingya accounts of atrocities, murder and sexual assaults.    The delegation called on the Myanmar government to allow access to press and international organizations so they could see the situation on the ground,  he said. U.S. State Department spokeswoman Heather Nauert said the issue has been taken to the highest level of authority in the U.S. government.  The statements related to the situation in Myanmar have been issued from the White House and also from the State Department.  She said the delegation would update U.S. Secretary of State Rex Tillerson before his visit to Myanmar on Nov. 15. Some 2,500 Rohingya refugees crossed into Bangladesh on Friday through two border crossings, a senior security official said on Saturday.  Those refugees are already being sent to different camps under the supervision of Bangladesh Army,  Major Mohammad Iqbal, Deputy Commanding Officer of Bangladesh Border Guard of Cox s Bazar district told Reuters. 