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Refugees fleeing Burma have accused security forces of burning and beheading Rohingya children in what they have described as “genocide” against the Muslim minority.

Violence erupted in the western Rakhine state just over a week ago when the government of Burma, also known as Myanmar, claimed it was cracking down on insurgents following an attack on police.

But the Rohingya accused security officials and Buddhist mobs of burning their villages down – leaving them forced to flee across the border to Bangladesh.

Around 400 people are believed to have been killed in the violent clashes, with a further 40,000 fleeing the country.

In the first six days after the August 25 attacks, the International Organisation for Migration said at least 18,000 Rohingya arrived in Bangladesh.

Myanmar’s military commander confirmed 399 people had been killed and said most of them were Rohingya insurgents – which they described as terrorists.

The government has accuses the insurgents of burning homes and killing members of the Buddhist ethnic Rakhine community.

But advocates for the Rohingya, an oppressed Muslim minority in overwhelmingly Buddhist Burma, said security forces and vigilantes attacked and burned Rohingya villages, shooting civilians and causing others to flee.

The insurgent group that claimed responsibility for last week's attacks, the Arakan Rohingya Salvation Army (ARSA), said it acted to protect Rohingya communities.

It is nearly impossible to verify information issued by either the government or Rohingya sympathisers because the government has barred most journalists from the area, except on limited official guided tours.

A human rights group, Fortify Rights, said that witnesses who escaped have supported accusations by Rohingya advocates that government security personnel and civilian vigilantes "committed mass killings of Rohingya Muslim men, women and children in Chut Pyin village, Rathedaung township, on August 27".

"Survivors and eyewitnesses from Chut Pyin told Fortify Rights that soldiers and armed residents burned every house in the village," the group said in a statement.

It quoted a 41-year-old survivor identified by the pseudonym Abdul Rahman as saying that soldiers killed and burned his brother along with other victims.

"We found (my other family members) in the fields," it quoted him as saying. "They had marks on their bodies from bullets and some had cuts. My two nephews, their heads were off. One was six years old and the other was nine years old. My sister-in-law was shot with a gun."

British Foreign Secretary Boris Johnson condemned the violence and said the treatment of the Rohingya was “besmirching the reputation of Burma.”

Mr Johnson appealed on the country’s leader Aung San Suu Kyi to step up and assist with peace-making efforts.

Most of Burma's estimated one million Rohingya live in the Rakhine state.

They face severe persecution, with the government refusing to recognise them as a legitimate native ethnic minority, leaving them without citizenship and basic rights.