The US ambassador to the UN urged Myanmar to intervene to stop communal violence there and protect humanitarian workers in the volatile western state of Rakhine.
Myanmar, formerly called Burma, has been shaken by religious unrest in recent years with at least 250 people killed in Buddhist-Muslim clashes since 2012.
Violence forced humanitarian workers to flee Rakhine earlier this month, leaving thousands facing looming food and water shortages.
"We continue to support Burma's reforms, but are greatly concerned that without effective government intervention violence in Rakhine could worsen, lives will be lost, and the critically needed humanitarian presence will not be sustainable," Samantha Power said.
"The government must take urgent steps to avoid more violence and to prevent setbacks on the journey to democracy and prosperity."
She spoke after the UN special adviser on Myanmar, Vijay Nambiar, updated the UN Security Council on the situation in the country's west.
The departure of humanitarian workers has aggravated the already critical health situation for hundreds of thousands of Rohingya Muslims dependent on international medical assistance, including 140,000 displaced people living in camps and more than 700,000 living in isolated villages.
Myanmar has promised to protect international aid groups targeted by Buddhist mobs, after a chorus of concern from foreign governments and the United Nations.
