Stories of horror from Myanmar's Rakhine State 04:40
Their homes burned and relatives killed, Rohingya have been fleeing northern Myanmar since October.
They trek for miles along a dangerous route -- risking drowning, disease and capture by the military -- to cross the border into neighboring Bangladesh, where refugee camps provide temporary shelter.
Tens of thousands of members of Myanmar's Muslim minority have left in this fashion, and their treatment may amount to "crimes against humanity," warns UN Special Rapporteur Yanghee Lee.
"When there's 77,000 people running away from their home towns, leaving everything ... the international community should really step up to the plate," she told CNN.
Lee has visited northern Rakhine State, which has been largely off limits to journalists and NGO workers since early October, and spoken to many refugees.
"What really struck me was when old men started to break down in tears in front of me and sob," she said, when interviewed on CNN's Newstream on Monday. "(They were) telling me they've seen their whole family killed in front of their eyes."
Myanmar government spokeswoman Aye Aye Soe said the administration is "deeply concerned by reports of potential human rights abuses and have already set up an Investigation Commission led by Vice President U Myint Swe."
Inside the Rohingya resistance 03:26
The Myanmar government did not respond to a request for comment about Lee's claims. In a 4,000-word statement given to CNN in February, Aye Aye Soe denied allegations of human rights abuses in Rakhine.
"What is happening in Rakhine now is only security clearance sweeps being carried out with restraint and within rules and regulations against armed perpetrators," she said. "The instigators are using this situation to portray a disproportionate picture of 'genocide' and 'ethnic cleansing'."
The violence began on October 9, when according to state media, a group of around 300 armed men attacked soldiers and police, sparking an intense crackdown by the Myanmar military.
"We attacked them using our machetes, swords, and knives, and we seized their weapons to use against them," Atah Ullah -- leader of the Harakat al-Yaqeen, or "Faith Movement," which carried out the attack -- told CNN earlier this year.
"We, the vulnerable and persecuted people, have asked the international community for protection against the atrocities by the government of Myanmar, but the international community turned its back on us," he said. "Finally, we cannot take it anymore."
16-month-old dies fleeing Myanmar violence 01:48
With the Myanmar military arresting hundreds of people in Rakhine, deploying attack helicopters and allegedly setting fire to villages, thousands of Rohingya refugees have poured over the border to camps in Bangladesh.
Amnesty International said it has documented "a wide range of human rights violations" since the crackdown began, while Human Rights Watch accused the military of "numerous abuses ... including widespread arson, extrajudicial killings, and systematic rape and other sexual violence."
In a piece for CNN Opinion in December, Matthew Smith of Thailand-based Fortify Rights warned that the world may be "watching a possible genocide unfold" in Rakhine.
"I would not use (the word genocide) right now but it could amount to crimes against humanity," said Lee.
She called on the international community to "walk the talk" on Myanmar, adding that Aung San Suu Kyi -- the country's long term democracy icon and de facto head of state -- "could and should speak up a little more."
Suu Kyi has been widely criticized for failing to address alleged human rights abuses in Rakhine. Suu Kyi has asked for more "time and space" to resolve the problems, and many have pointed out that the country's military, rather than the civilian government, oversees the region.
"It is important that the Investigation Commission is allowed to complete its work, and to demonstrate that Myanmar's own institutions are able to address our human rights challenges," Aye Aye Soe said.
"In the meantime, we will continue to seek long term solutions to bring about lasting peace and stability in Rakhine."
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