US Secretary of State Rex Tillerson says Myanmar’s military action against the minority Muslim Rohingya population constitutes ethnic cleansing.
He said the Rohingya had suffered “horrendous atrocities” and as a result the US was considering targeted sanctions against those responsible.
In September, UN human rights chief Zeid Raad Al Hussein also accused Myanmar of ethnic cleansing.
Mr Tillerson’s comments come days before a visit to Myanmar by the Pope.
The Rohingya are a stateless minority who have long experienced persecution in Myanmar, also known as Burma. More than 600,000 have fled to neighbouring Bangladesh since violence erupted in Rakhine state late in August.
“After a careful and thorough analysis of available facts, it is clear that the situation in northern Rakhine state constitutes ethnic cleansing against the Rohingya,” Mr Tillerson said in a statement.
“These abuses by some among the Burmese military, security forces, and local vigilantes have caused tremendous suffering and forced hundreds of thousands of men, women, and children to flee their homes in Burma to seek refuge in Bangladesh,” he said.
He said the US wanted an independent investigation into events in Rakhine state and targeted sanctions could follow.
“Those responsible for these atrocities must be held accountable,” he added.
Mr Tillerson’s comments reflect a hardening US stance on Myanmar in recent days. On a one-day visit to the country last week he said Washington was “deeply concerned” by reports of widespread atrocities.
He urged the government and security forces to respect the human rights of all those within its borders.
US builds pressure on Myanmar
Analysis by Barbara Plett Usher, US state department correspondent
This is the strongest US condemnation yet of the crackdown against the Rohingya.
Two months ago a senior UN official talked about a textbook case of ethnic cleansing, but the US has been slow to follow suit.
Now Mr Tillerson says the state department has come to the same conclusion after careful analysis. The designation does not have any legal consequences but Mr Tillerson said the US was considering targeted sanctions against those responsible.
However he has rejected broad-based sanctions as a counterproductive approach that wouldn’t resolve the complex and long-running ethnic tensions at the root of the crisis.
A senior state department official said the designation was aimed at increasing pressure on Myanmar authorities to repatriate the refugees and make it safe for them to return.
The tougher US rhetoric comes after a delegation, led by Senator Jeff Merkley, recently returned from a visit to Myanmar and Bangladesh.
Members of the delegation said they were disturbed by the reports of attacks on Rohingya, including the rape and murder of family members.
The BBC has also heard numerous first-hand reports from refugees of killings, rape and even massacres. Inside Rakhine state itself, a BBC crew witnessed burned out homes and a Muslim village being set on fire.
Despite widespread accusations of human rights violations, Myanmar has consistently denied persecuting its Rohingya minority.
Earlier this month, the Myanmar army released the results of an internal investigation in which it exonerated itself of blame for the crisis.
It said “terrorists” from within the Rohingya community (which it called Bengali) were responsible for burning houses and that those who fled did so because they feared those terrorists.
Amnesty International dismissed the report as an attempted “whitewash”.
Amid growing international concern over the crisis, China recently proposed a three-stage plan starting with a ceasefire to allow refugees to return to Myanmar.
China’s Foreign Minister Wang Yi said talks should follow and the final phase would be a long-term solution to end poverty among the Rohingya.
Beijing has close ties with both Myanmar and Bangladesh and in the past has had to deal with thousands of refugees seeking shelter in China after fleeing lawless border regions.
Pope Francis is due to arrive in Myanmar on 26 November. His visit will include meetings with army chief General Min Aung Hlaing and the country’s de-facto civilian leader, Aung San Suu Kyi, the Vatican has said.