The United Nations human rights office on Friday warned there was a risk of a serious rise in violence in Myanmar's Rakhine state, where it received "frightening and disturbing reports" of the killing of Rohingya people and burning of their property.
The United States, the European Union, Britain and other nations expressed alarm at the escalating conflict in a separate statement highlighting the increasing harm to civilians.
A powerful armed ethnic group in Myanmar known as the Arakan Army said last week it had won control over a town in the western state of Rakhine after weeks of fighting.
It has denied accusations it had targeted members of the Muslim-minority Rohingya during the offensive.
The Office of the United Nations High Commissioner for Human Rights (OHCHR) said that tens of thousands of civilians had been displaced by the fighting in the townships of Buthidaung and Maungdaw, with an estimated 45,000 Rohingya fleeing near the border with Bangladesh to seek protection.
"We see clear and present risks of a serious expansion of violence as the battle for neighbouring Maungdaw town has begun - where the military maintains outposts and where a large Rohingya community lives, including hundreds of displaced Rohingya who moved to town from villages seeking safety," said OHCHR spokesperson Elizabeth Throssell.
"The High Commissioner calls for an immediate end to the violence, and for all civilians to be protected without any distinction based on identity."
Rohingya have faced persecution in Buddhist-majority Myanmar for decades. After escaping a military-led crackdown in 2017, nearly a million of them live crammed into refugee camps in Bangladesh's border district of Cox's Bazar.
In a joint statement, the United States, the European Union, Britain, Canada, Australia and other nations cited credible reports of abuses committed against the civilian population.
"In Rakhine state, towns and villages have been consistently targeted by the military regime and armed groups ... all populations are facing extreme levels of food insecurity," they said. "There must be accountability for all atrocities committed in Myanmar."