Maungdaw, Myanmar: At least 71 people including 12 security forces were killed as Rohingya militants besieged border posts in northern Rakhine State, Myanmar's authorities said Friday, triggering a fresh exodus of refugees towards Bangladesh. 

The state is bisected by religious hatred focused on the stateless Rohingya Muslim minority, who are reviled and perceived as illegal immigrants in Buddhist-majority Myanmar.  

The office of de-facto leader Aung San Suu Kyi said 12 security officials had been killed alongside 59 militants -- the highest declared single day toll since fighting broke out last year.

Friday's fighting exploded around Rathedaung township which has seen a heavy build-up of Myanmar troops in recent weeks, with reports filtering out of killings by shadowy groups, army-blockaded villages and abuses. 

Some 20 police posts came under attack in the early hours of Friday by an estimated 150 insurgents, some carrying guns and using homemade explosives, Myanmar's military said.

"The military and police members are fighting back together against extremist Bengali terrorists," Commander-in-Chief Min Aung Hlaing said in a statement on Facebook, using the state's description for Rohingya militants.

One resident in Maungdaw, the main town in northern Rakhine, said gunfire could be heard throughout the night. 

"We are still hearing gunshots now, we dare not to go out from our house," the resident said by phone, asking not to be named. 

Footage obtained by AFP showed smoke rising from Zedipyin village in Rathedaung township where fighting was ongoing Friday.

- Rohingya militancy -

Despite years of persecution, the Rohingya largely eschewed violence. 

But a previously unknown militant group emerged as a force last October under the banner of the Arakan Rohingya Salvation Army (ARSA), which claims to be leading an insurgency based in the remote May Yu mountain range bordering Bangladesh.

A Twitter account (@ARSA_Official) which frequently posts purportedly from the group confirmed its fighters were engaging Myanmar's military in the area and accused the soldiers of carrying out atrocities in recent weeks.

Myanmar says the group is headed by Rohingya jihadists who were trained abroad but it is unclear how large the network is. 

Suu Kyi's office posted pictures of weapons that had been taken from militants, mainly home-made bombs and rudimentary knives and clubs.

Friday's violence pushed new waves of Rohingya to flee towards Bangladesh. 

But border guards there said they would not be allowed to cross. 

"More than a thousand of Rohingya women along with children and cattle have gathered near the land border between Myanmar and Bangladesh since this morning," Manjurul Hasan Khan, commander of Ukhiya town's border guards, told AFP.

The flare-up came just hours after former UN chief Kofi Annan released a milestone report detailing conditions inside Rakhine and offering ways to heal the festering sectarian tensions there. 

Commissioned by Myanmar's own government, it urged the scrapping of restrictions of movement and citizenship imposed on the roughly one million-strong Rohingya community in Rakhine. 

In a statement Annan said he was "gravely concerned" by the latest outbreak of fighting. 

"The alleged scale and gravity of these attacks mark a worrying escalation of violence," he said. 

- New crackdown fears -

The UN's top official in Myanmar, Renata Lok-Dessallien, called on all sides to "refrain from violence, protect civilians (and) restore law and order".

The wedge of Rakhine closest to Bangladesh has been in lockdown since October 2016.

Deadly attacks by the militants on border police sparked a military response that left scores dead and forced some 87,000 people to flee to Bangladesh.

The UN believes the military crackdown may have amounted to ethnic cleansing of the Rohingya. 

But the army and Aung San Suu Kyi's civilian government vehemently deny allegations of widespread abuses, including rapes and murders.

They have so far refused to grant visas to UN investigators tasked with probing the allegations. 

Amnesty International said there were now fears over how Myanmar's notoriously abusive security forces might respond. 

"This cannot lead to repeat of last year's vicious military reprisals responding to a similar attack, when security forces tortured, killed and raped Rohingya people and burned down whole villages," said Amnesty's regional campaigns director Josef Benedict. 

Myanmar security forces have conducted sporadic operations to flush out suspected militants this year, often resulting in casualties among Rohingya villagers.

They have spoken of their fear at being trapped in between security forces and the militants, who are accused of conducting a shadowy assassination campaign against perceived collaborators with the state.

Access to the area is severely restricted and verifying information is difficult. 

Activists and supporters on both sides of the sectarian divide have a history of posting false images and footage online.