Allegations that Myanmar soldiers are killing, raping and torturing villagers in Rakhine, a restive region that is home to the persecuted Muslim Rohingya, must be independently investigated, rights groups said.
Northern Rakhine has been under a military lockdown since an attack on border guards three weeks ago left nine policeman dead.
The government has blamed the raids on Rohingya militants and a search for the culprits has seen more than 30 people killed and dozens arrested, according to official reports.
Stories of grave abuse by security officers -- including sexual violence, summary executions and the torching of villages -- have spiralled on social media but are difficult to verify with the army barring rights groups and journalists from the remote region bordering Bangladesh.
On Friday, Amnesty International and Human Rights Watch joined calls for an impartial investigation into the allegations, which the UN has called "alarming and unacceptable".
"If Myanmar's security forces are not involved in any human rights violations as the authorities claim, then they should have no trouble granting independent observers access," said Rafendi Djamin, Amnesty's Southeast Asia and Pacific director.
The same day, the US State Department said that it had voiced concern to Myanmar's foreign ministry about the reported rape of Rohingya Muslim women by soldiers during a recent upsurge in violence against the persecuted minority.
State Department spokesman Mark Toner told a briefing that the United States wanted Myanmar to investigate the reported rapes and hold those responsible accountable.
Toner said earlier that the United States had raised the issue with the Myanmar foreign minister, but the State Department later issued a transcript of the briefing saying it had brought the issue up with "the foreign ministry," not the foreign minister.
Eight Rohingya women, all from U Shey Kya village in Rakhine State, described how soldiers last week raided their homes, looted property and raped them at gun point.
Reuters interviewed three of the women in person and five by telephone, and spoke with human rights groups and community leaders. Not all the claims could be independently verified, including a total number of women assaulted.
Zaw Htay, a spokesman for Myanmar President Htin Kyaw, denied the allegations. The military did not respond to an emailed request for comment about the accusations.
Writing on Facebook, Zaw Htay dismissed an article in the Myanmar Times that described reports of a "mass rape" in a Rohingya village on October 19.
"There was information that some attackers were kept in that village. So security was taken very seriously and (the search team) was very careful about being safe and would not think to rape up to 5 women," he wrote.
The government says the October 9 border raids were carried out by hundreds of Rohingya fighters linked to Taliban-trained Islamists.
If true, it would mark a troubling development in a religiously-split region where the stateless Rohingya have languished under years of repression but so far shown little interest in jihadist ideology.
Rakhine has sizzled with tension ever since waves of communal violence in 2012 killed more than 100 and pushed tens of thousands of people, mostly Rohingya, into destitute displacement camps.
Many in Buddhist-majority Myanmar insist the Rohingya are illegal immigrants from Bangladesh and viscerally oppose any moves to grant them citizenship.
The recent upsurge in violence deepens and complicates a conflict that already posed a top challenge to a new civilian government led by Aung San Suu Kyi, who has disappointed rights groups by not coming out in stronger support of the Rohingya.