The United States has declared the violence against Rohingya Muslims in Burma to be “ethnic cleansing”, putting more pressure on the country's military to halt a crackdown that has sent more than 600,000 refugees flooding over the border to Bangladesh.

Secretary of State Rex Tillerson blamed Burma's security forces and “local vigilantes” for what he called “intolerable suffering” by the Rohingya. Although the military has blamed Rohingya insurgents for setting off the crisis, Mr Tillerson said that “no provocation can justify the horrendous atrocities that have ensued.”

“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.

Those who perpetrated the atrocities “must be held accountable,” Mr Tillerson said. He added that the US wanted a full investigation and would seek justice “through US law, including possible targeted sanctions.”

The declaration followed a lengthy review process by the administration of President Donald Trump to determine whether the violence met the threshold to be considered ethnic cleansing. The United Nations came to that conclusion in September, but the U.S. had held off, with Tillerson saying he needed more information even as he expressed deep concern about the crisis.

Rohingya from Burma's Rakhine state have been fleeing to neighbouring Bangladesh, seeking refuge from what Burma's military has called “clearance operations.” The crisis started in August, when Rohingya insurgents attacked Burma security forces, leading to a brutal crackdown in which soldiers and Buddhist mobs have faced allegations of killing men, raped woman and burned homes and property to force the Rohingya to leave.

Rohingya refugees – in pictures Show all 15 1 /15 Rohingya refugees – in pictures Rohingya refugees – in pictures A young girl and a baby wade through mud after arriving in Whaikhyang, Bangladesh from Burma on 10 September Dan Kitwood/Getty Images Rohingya refugees – in pictures Rohingya refugees walk through a camp in Whaikhyang, Bangladesh after arriving from Burma Dan Kitwood/Getty Images Rohingya refugees – in pictures A young Rohingya refugee gathers firewood after arriving in Whaikhyang, Bangladesh from Burma Dan Kitwood/Getty Images Rohingya refugees – in pictures Rohingya refugees wait for sacks of rice to be distributed in Whaikhyang, Bangladesh Dan Kitwood/Getty Images Rohingya refugees – in pictures Rohingya Muslim refugees arrive on a boat in Whaikhyang, Bangladesh after crossing from Burma on 8 September Dan Kitwood/Getty Images Rohingya refugees – in pictures Rohingya Muslim refugees react after being re-united with each other after arriving in Whaikhyang, Bangladesh on a boat from Burma Getty Rohingya refugees – in pictures Rohingya Muslim refugees walk along the remains of a road after arriving in Whaikhyang, Bangladesh on a boat from Burma Dan Kitwood/Getty Images Rohingya refugees – in pictures Rohingya Muslim refugees wade through water after arriving in Whaikhyang, Bangladesh by boat from Burma Dan Kitwood/Getty Images Rohingya refugees – in pictures Rohingya Muslim refugees wade through water after arriving in Whaikhyang, Bangladesh by boat from Myanmar Dan Kitwood/Getty Images Rohingya refugees – in pictures Rohingya Muslim refugees stand in the rain after arriving in Whaikhyang, Bangladesh by boat from Burma Dan Kitwood/Getty Images Rohingya refugees – in pictures Indian children hold placards and shout slogans during a protest against the alleged persecution of the Rohingya Muslims in Burma EPA/Raminder Pal Singh Rohingya refugees – in pictures Supporters of the Difa-e-Pakistan Council (DPC), an Islamic organisation, listen to their leaders' speeches against Burma's persecution of Rohingya Muslims, during a demonstration in Karachi Reuters/Akhtar Soomro Rohingya refugees – in pictures Hundreds of Iranians take part in a protest against violence in Myanmar after weekly Friday prayers, in Tehran EPA/Abedin Taherkenareh Rohingya refugees – in pictures Indonesian Muslim activists hold placards and shout slogans during a protest against the alleged persecution of the Rohingya minority in Magelang, Central Java, Indonesia EPA/Ali Lutfi Rohingya refugees – in pictures Members of an Islamic organisation shout slogans against the Burma government during a protest in Dhaka, Bangladesh EPA

Last week, Mr Tillerson travelled to Burma in the highest level visit by a US official since Mr Trump took office. US officials dangled the possibility of an “ethnic cleansing” designation ahead of Mr Tillerson's trip, potentially giving him more leverage as he met with Burmese officials. In the capital of Naypitaw, Mr Tillerson met with the country's civilian leader, Aung San Suu Kyi, well as the Myanmar's powerful military chief, Min Aung Hlaing, who is in charge of operations in Rakhine state.

Although the “ethnic cleansing” label doesn't carry specific legal requirements for the US, it is likely to intensify calls for the Trump administration and Congress to move toward new sanctions on Burma. Sanctions on the Southeast Asian nation were eased under former President Barack Obama as the country made steps toward transitioning to democracy.

Pressure from Congress to take punitive steps against Burma has been mounting. Earlier this month, the House passed a non-binding resolution condemning “murderous ethnic cleansing and atrocities against civilians.” It called on Trump to impose sanctions on those responsible for human rights abuses, including members of Burma's military and security services.

Mr Tillerson, during his visit to Burma, said the U.S. would consider targeted sanctions against individuals deemed responsible for the violence, but that he wasn't advocating “broad-based economic sanctions” against the entire nation.

Drone footage shows thousands of Rohingya Muslims fleeing Myanmar

US officials have been concerned that pushing Burma's leaders too hard on the Rohingya violence could undermine the country's civilian government, led for the last 18 months by Ms Suu Kyi. That could slow or reverse the country's delicate transition away from decades of harsh military rule, and also risks pushing Burma away from the U.S. and closer to China.

The State Department has also examined whether the violence in Rakhine meets the definitions for “crimes against humanity” or “genocide,” but have thus far made no such determinations.

According to the United Nations Office on Genocide Prevention, “ethnic cleansing” isn't recognised as an independent crime under international law, unlike crimes against humanity and genocide. It surfaced in the context of the 1990s conflict in the former Yugoslavia, when a U.N. commission defined it as “rendering an area ethnically homogeneous by using force or intimidation to remove persons of given groups from the area.”

Human rights groups accuse the military of a scorched-earth campaign against the Rohinyga, who numbered roughly one million in Myanmar before the latest exodus. The Buddhist majority in Burma believes they migrated illegally from Bangladesh, but many Rohingya families have lived for generations in Burma. In 1982, they were stripped of their citizenship.

Already, the United States has curtailed its ties to Burma's military over the violence. Earlier this year the US restored restrictions on granting visas to members of Myanmar's military, and the State Department has deemed units and officers involved in operations in Rakhine state illegible for US assistance.