Myanmar's military carried out mass killings and gang rapes of Muslim Rohingya with "genocidal intent" and the commander-in-chief and five generals should be prosecuted for orchestrating the gravest crimes under law, UN investigators said on Monday.
The civilian government led by Aung San Suu Kyi has allowed hate speech to thrive, destroyed documents and failed to protect minorities from crimes against humanity and war crimes by the army in Rakhine, Kachin and Shan states, they said in a report.
In doing so, it "contributed to the commission of atrocity crimes", the report said.
A year ago, government troops led a brutal crackdown in Myanmar's Rakhine state in response to attacks by the Arakan Rohingya Salvation Army on 30 Myanmar police posts and a military base.
Some 700,000 Rohingya fled the crackdown and most are now living in refugee camps in neighbouring Bangladesh.
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The UN report said the military action, which included the scorching of villages, was "grossly disproportionate to actual security threats".
The United Nations defines genocide as acts meant to destroy a national, ethnic, racial or religious group in whole or in part. Such a designation is rare under international law, but has been used in countries including Bosnia and Sudan and in the Islamic State campaign against the Yazidi communities in Iraq and Syria.
"The crimes in Rakhine State, and the manner in which they were perpetrated, are similar in nature, gravity and scope to those that have allowed genocidal intent to be established in other contexts," said the UN Independent International Fact-Finding Mission on Myanmar.
In the final 20-page report, it said: "There is sufficient information to warrant the investigation and prosecution of senior officials in the Tatmadaw (army) chain of command, so that a competent court can determine their liability for genocide in relation to the situation in Rakhine state."
The Myanmar government, which was sent an advance copy of the UN report in line with standard practice, has not commented.
Contacted by phone, Myanmar military spokesman Major General Tun Tun Nyi said he could not immediately comment.
The UN panel, led by former Indonesian attorney-general Marzuki Darusman, named the Myanmar army's commander-in-chief, Senior General Min Aung Hlaing, and five other generals who should face justice.
They included Brigadier-General Aung Aung, commander of the 33rd Light Infantry Division, which oversaw operations in the coastal village of Inn Din where 10 Rohingya captive boys and men were killed.
Reuters was unable to contact Min Aung Hlaing or Aung Aung on Monday.
The massacre was uncovered by two Reuters journalists - Wa Lone, 32, and Kyaw Soe Oo, 28 - who were arrested as a result last December and are being tried on charges of violating the country's Official Secrets Act.
The court had been due to deliver its verdict on Monday, but at a brief hearing earlier the proceedings were postponed until Sept. 3.
In April, seven soldiers were sentenced to 10 years in prison with hard labour for participating in the massacre.
The report said Suu Kyi, a Nobel Peace Prize laureate, "has not used her de facto position as Head of Government, nor her moral authority, to stem or prevent the unfolding events, or seek alternative avenues to meet a responsibility to protect the civilian population".
Suu Kyi's spokesman, Zaw Htay, could not immediately be reached for comment on Monday.
The top UN human rights official Zeid Ra'ad al-Hussein has called the crackdown against the Rohingya a "textbook example of ethnic cleansing".
Suu Kyi's government has rejected most allegations of atrocities made against the security forces by refugees. It has built transit centres to receive Rohingya returnees to western Rakhine state, but UN aid agencies say that it is not yet safe for them to return.
The UN Security Council should ensure all perpetrators are held to account, preferably by referring Myanmar to the International Criminal Court or by creating an ad hoc tribunal, the investigators said.
The Security Council should "adopt targeted individual sanctions, including travel bans and asset freezes, against those who appear most responsible for serious crimes under international law" and impose an arms embargo on Myanmar, they said.
The four other generals the UN panel said should be prosecuted were named as the army deputy commander-in-chief, Vice Senior-General Soe Win; the commander of the Bureau of Special Operations-3, Lieutenant-General Aung Kyaw Zaw; the commander of Western Regional Military Command, Major-General Maung Maung Soe; and the commander of 99th Light Infantry Division, Brigadier-General Than Oo.
Reuters was not able to contact those four generals on Monday.
The panel, set up last year, interviewed 875 victims and witnesses in Bangladesh and other countries, and analysed documents, videos, photographs and satellite images.
Decades of state-sponsored stigmatisation against Rohingya had resulted in "institutionalised oppression from birth to death", the report said.
The Rohingya, who regard themselves as native to Rakhine state, are widely considered as interlopers by Myanmar's Buddhist majority and are denied citizenship.
"The Tatmadaw acts with complete impunity and has never been held accountable. Its standard response is to deny, dismiss and obstruct," the UN report said.
The report also criticised Facebook's response to allegations, including by members of the same UN panel in March, that the social media giant had been used to incite violence and hatred against the Rohingyas.
"Although improved in recent months, Facebook's response has been slow and ineffective. The extent to which Facebook posts and messages have led to real-world discrimination and violence must be independently and thoroughly examined," it said.
Facebook declined to comment in an emailed statement, saying it had not yet seen the report.
Facebook said in a statement issued 10 days ago following a Reuters investigative report into its failure to combat hate speech against the Rohingya and other Muslims that it had been "too slow" to address the problem in Myanmar and was acting to remedy the situation by hiring more Burmese speakers and investing in technology to identify problematic content.
International courts have a mixed record on prosecutions for genocide.
In 2008, a UN court sentenced former army colonel Theoneste Bagosora, accused of masterminding the slaughter of 800,000 people in Rwanda in 1994, to life in prison on charges of genocide and crimes against humanity.
His sentence was later cut to 35 years on appeal.
In 2016, former Bosnian Serb leader Radovan Karadzic was convicted by
U.N. judges of genocide for the 1995 Srebrenica massacre. He is appealing against the conviction.
The ICC issued arrest warrants for Sudan President Omar al-Bashir in 2009 and 2010 over his alleged role in war crimes including genocide in Sudan's breakaway Darfur province in 2003. He remains in office.