The statement from Secretary of State Rex Tillerson, who visited Myanmar last week, is the strongest US condemnation yet of the military's crackdown against the Rohingya, which has triggered a major refugee crisis and escalating global outrage.
"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," Tillerson said in a statement.
"No provocation can justify the horrendous atrocities that have ensued."
More than 600,000 Rohingya have fled mainly Buddhist Myanmar for Bangladesh since the military launched a counter-insurgency operation in troubled Rakhine state in late August.
While the army insists it has only targeted Rohingya rebels, refugees massing in Bangladeshi camps have given chilling and consistent accounts of widespread murder, rape and arson at the hands of security forces and Buddhist mobs.
"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," Tillerson said.
Myanmar's response to the crisis will be vital to determining the success of its transition to becoming "a more democratic society," he added.
Suu Kyi's role
Myanmar's de facto civilian leader Aung San Suu Kyi – a Nobel peace laureate – has been criticized by rights groups disappointed with her failure to condemn the crackdown or publicly criticize the military.
Washington says Suu Kyi has a crucial role to play in tackling the crisis but has been careful to focus blame on the army.
On his one-day visit to Myanmar's capital Naypyidaw, Tillerson said Washington was “deeply concerned by credible reports of widespread atrocities committed by Myanmar's security forces and vigilantes”.
He urged Myanmar to accept an independent investigation into those allegations, after which individual sanctions could be appropriate.
On Wednesday, Tillerson said: "Burma's government and security forces must respect the human rights of all persons within its borders, and hold accountable those who fail to do so."
Last year, former US president Barack Obama eased long-standing sanctions on Myanmar, reinstating preferential tariffs, to help the country's transition from being a military-run pariah to civilian government.
The current administration says it does not favour a return to broad economic sanctions.
But a senior State Department official told reporters on Wednesday: "We are looking at additional sanctions targeting individuals responsible for specific acts of violence."
Myanmar's army and Suu Kyi's administration have dismissed reports of atrocities and refused to grant entry to UN investigators tasked with probing alleged abuses.
Some world leaders had already described the scorched-earth military campaign against the Rohingya as "ethnic cleansing."
Global human rights watchdogs over the past week issued reports accusing Myanmar security forces of slitting the throats of Rohingya, burning victims alive and gang-raping women and girls.
The watchdogs described "mounting evidence" of genocide and spoke of "ethnic cleansing" founded on years of “apartheid”.
Uptick in diplomacy
Diplomatic efforts to resolve the crisis have intensified.
France's Foreign Minister Jean-Yves Le Drian and the European Union's top diplomat, Federica Mogherini, both visited Myanmar on Monday.
China has made a proposal to resolve the crisis with a ceasefire, refugee repatriation and poverty alleviation.
A foreign ministry spokesman in Beijing said Monday that the plan had won approval from leaders in both Bangladesh and Myanmar.
"We understand that both Burma and Bangladesh are close to reaching an agreement on a process for voluntary repatriations," the senior State Department official said, adding that by declaring "ethnic cleansing," the US hopes to increase the pressure for such a deal.
US Senator John McCain, who chairs the Senate Armed Services Committee, welcomed Tillerson's statement as a first step that should be followed by "targeted sanctions against the military officials responsible for these atrocities".
