Myanmar is committing crimes against humanity in its campaign against Muslim insurgents in Rakhine state, Human Rights Watch said Tuesday, and called for the UN Security Council to impose sanctions and an arms embargo.

A government spokesman rejected the accusation, saying there was no evidence and the government was committed to protecting rights.

Myanmar has also rejected UN accusations that its forces are engaged in ethnic cleansing against Rohingya Muslims in response to co-ordinated attacks by Rohingya insurgents on the security forces on Aug. 25. It says its forces are fighting terrorists responsible for attacking the police and the army, for killing civilians and for torching villages.

The military campaign has sent 480,000 refugees fleeing to Bangladesh since Aug. 25, most of them Rohingya. International aid bodies said on Tuesday the figure had risen because 35,000 new arrivals had not been included in a previous tally.

Human Rights Watch have accused the security forces and Buddhist vigilantes of trying to drive Rohingya out of Buddhist-majority Myanmar.

"The Burmese military is brutally expelling the Rohingya from northern Rakhine state," said James Ross, legal and policy director at Human Rights Watch.

"The massacres of villagers and mass arson driving people from their homes are all crimes against humanity," he said.

The International Criminal Court defines crimes against humanity as acts including murder, torture, rape and deportation "when committed as part of a widespread or systematic attack directed against any civilian population, with knowledge of the attack."

Human Rights Watch said its research, supported by analysis of satellite imagery, had found crimes of deportation and forced population transfers, murder and attempted murder, rape and other sexual assault and persecution.

​The UN Security Council and concerned countries should impose targeted sanctions and an arms embargo, the group said.

Government spokesperson Zaw Htay rejected the accusation, saying no Myanmar government was ever as committed to the promotion of rights as much as the current one.

"Accusations without any strong evidence are dangerous. It makes it difficult for the government to handle things," he told Reuters.

Campaign called 'disporportionate'

The violence in Rakhine state and the refugee exodus represent the biggest crisis the government of Nobel laureate Aung San Suu Kyi has faced since it came to power last year in a transition from nearly 50 years of harsh military rule.

Myanmar regards the Rohingya as illegal immigrants from Bangladesh and communal violence has flared periodically for decades.

Rohingya refugees clamour for supplies in a crowded wait for aid Sunday. (Cathal McNaughton/Reuters)

Most Rohingya are stateless. For years, the United States and its Western allies imposed sanctions on Myanmar in support of Suu Kyi's campaign for democracy. Its response was to forge closer ties with China.

The United States has criticized the military campaign as "disproportionate" and called for an end to violence, but a Trump administration official said this month he did not expect a return to sanctions.

Suu Kyi has faced unprecedented criticism and calls for her Nobel prize to be withdrawn. She has denounced rights violations and vowed that abusers would be prosecuted. Suu Kyi has little if any control over the security forces under a military-drafted Constitution that also bars her from the presidency and gives the military veto power over political reform.

Insurgent groups denies killings

Myanmar has seen a surge of Buddhist nationalism over recent years, and the public is supportive of the campaign against insurgents blamed for starting the violence. Since Sunday, the army has unearthed the bodies of 45 members of Myanmar's small Hindu community who authorities say were killed by the insurgents soon after the violence erupted.

The Arakan Rohingya Salvation Army insurgent group, which has claimed attacks on the security forces since October, denied killing the villagers. Rights groups have said they have found evidence of abuses by the insurgents, including the burning of some Buddhists' homes, but on a much smaller scale than abuses by the army and Buddhist vigilantes.

Some Hindus have fled to Bangladesh, complaining of violence against them by soldiers or Buddhist vigilantes. Others have taken refuge in Myanmar towns, accusing the insurgents of attacking them on suspicion of being government spies.