Medecins Sans Frontieres (Doctors Without Borders) has said it has been expelled from Myanmar and that tens of thousands of lives are at risk in the country.
The decision came after the humanitarian group reported it recently treated nearly two dozen Rohingya Muslim victims of communal violence in Rakhine state, which the government has denied.
The Nobel Prize-winning aid group said it was "deeply shocked" by Myanmar's decision to expel it after two decades of work in the country, the AP news agency reported.
The US said it was very concerned and urged the government to continue to provide "unfettered" access for humanitarian agencies.
"Today for the first time in MSF's history of operations in the country, HIV/AIDS clinics in Rakhine, Shan and Kachin states, as well as Yangon division, were closed and patients were unable to receive the treatment they needed," MSF said in a statement, using the French acronym for its name.
It said it had to close clinics serving 30,000 HIV/AIDS patients, and more than 3,000 people with tuberculosis would not be able to get vital medicine, the Reuters news agency reported.
Myanmar's presidential spokesman Ye Htut had criticised MSF in the Myanmar Freedom newspaper for hiring "Bengalis," the term the government uses for the Rohingya minority, and lacked transparency in its work.
He also accused the group of misleading the world about an attack last month in the remote northern part of Rakhine.
The UN says more than 40 Rohingya may have been killed, but the government has vehemently denied allegations that a Buddhist mob rampaged through a village, killing women and children.
It says one policeman was killed by Rohingya and no other violence occurred.
MSF said it treated 22 injured and traumatised Rohingya.
Repeated attempts to reach Ye Htut for comment were unsuccessful Friday, AP reported.
Myanmar, a predominantly Buddhist nation of 60 million, only recently began to emerge from a half-century of military rule.
Since then, ethnic tensions have swept Rakhine state, raising concerns from the US and others that the bloodshed could undermine democratic reforms.
Up to 280 people have been killed and tens of thousands more have fled their homes, most of them Rohingya.
Since the violence erupted in June 2012, MSF has worked in 15 camps for the displaced people in Rakhine state.
For many of the sickest patients, the organisation offers the best and sometimes only care, because travelling outside the camps for treatment in local Buddhist-run hospitals can be dangerous and expensive.
The aid group has worked to help smooth the referral process for emergency transport from some camps.
Due to increasing threats and intimidation from a group of Rakhine Buddhists who have been holding near daily protests against MSF, the organisation has said its activities have been severely hampered and that it has not received enough government support, AP reported.
"We urge the government to continue to work with the international community to provide humanitarian assistance to communities in need and to unsure unfettered access for humanitarian agencies," US State Department spokeswoman Jen Psaki told reporters in Washington.
Democratic Representative Joe Crowley, one of the most prominent voices in the US Congress on Myanmar, also reacted to the reported expulsion.
"It is the responsibility of the Burmese government to protect civilians. This is deeply troubling," he said in a tweet.
"MSF's actions are guided by medical ethics and the principles of neutrality and impartiality," MSF said in its statement read.
"MSF is in discussions with the Government of Myanmar to allow our staff to resume life-saving medical activities across the country and continuing addressing the unmet heath needs of its people."