At least 32 people killed on Friday after insurgents attacked 24 police posts.
ARSA, group that staged October attacks, claims responsibility.
Army sources estimate 1,000 insurgents from 50 villages involved.
At least 21 insurgents and 11 members of the security forces were killed in Myanmar's troubled Rakhine state on Friday when militants staged a major coordinated attack on 24 police posts and an army base, the military said.
The fighting - still going on in some areas - marked a major escalation in a simmering conflict in the northwestern state since last October, when similar attacks prompted a big military sweep beset by allegations of serious human rights abuses.
The Arakan Rohingya Salvation Army (ARSA), a group previously known as Harakah al-Yaqin, or "Faith Movement", which instigated the October attacks, claimed responsibility for the early morning offensive, and warned of more attacks.
The treatment of approximately 1.1 million Muslim Rohingya has emerged as majority Buddhist Myanmar's most contentious human rights issue as it makes a transition from decades of harsh military rule.
It now appears to have spawned a potent insurgency, which has grown in size and morphed from an militant group into something more akin to a widespread movement, observers say.
The situation in the state deteriorated early this month when security forces began a new "clearance operation" in a remote mountain area.
The army said that one soldier, 10 police and 21 insurgents had been killed in the attacks. Two military sources contacted by Reuters said there may be more deaths.
"In the early morning at 1 a.m., the extremist Bengali insurgents started their attack on the police post ... with the man-made bombs and small weapons," said the army, referring to the Rohingya with the derogatory term implying they are interlopers from Bangladesh.
It added the militants also used sticks and swords and destroyed bridges with explosives. Further attacks came at around 3 a.m. and 4 a.m., it said.
The Rohingya are denied citizenship and are seen by many in Myanmar as illegal immigrants from Bangladesh, despite claiming roots in the region that go back centuries, with communities marginalized and occasionally subjected to communal violence.
The military counter-offensive in October resulted in some 87,000 Rohingya fleeing to Bangladesh, where they joined many others who have fled from Myanmar over the past two decades or more.
The United Nations said Myanmar's security forces likely committed crimes against humanity in the offensive that began in October.
The military said about 150 Rohingya attacked an army base in Taung Bazar village in Buthidaung township, prompting the army to fight back.
"They were planning to attack because we have found their camps, the caves and the bombs and masks inside the caves," said Myanmar police spokesman Colonel Myo Thu Soe.
The morning attack took place hours after a panel led by the former U.N. chief Kofi Annan advised the government on long-term solutions for the violence-riven state.
Annan condemned the violence on Friday, saying that the "alleged scale and gravity of these attacks mark a worrying escalation of violence. No cause can justify such brutality and senseless killing".
Military sources in Rakhine told Reuters they estimated the number of insurgents in the offensive stood at about 1,000 and that it encompassed both Maungdaw and Buthidaung townships - a much wider area compared with October.
The leader of ARSA, Ata Ullah, has said hundreds of young Rohingya have joined the group, which claims to be waging a legitimate defence against the army and for human rights.
"We have been taking our defensive actions against the Burmese marauding forces in more than 25 different places across the region. More soon!" the group said on a Twitter account believed to be linked to it.
Over the last several months the government has accused the insurgents of instigating a campaign of terror against village chiefs and killing government informers, disrupting government information networks.
"The insurgents were able to organize such a huge attack because they were successful in organizing the information blackout," said one of the military sources.
"It's an extremely serious situation which could lead to a major crisis.
It's not easy to know what has happened, but it's clearly very worrying," said Yangon-based analyst and former U.N. diplomat in Myanmar Richard Horsey.
Bangladesh pushed back around 146 Rohingya trying to flee following the violence, S.M. Ariful Islam, a Border Guard Bangladesh officer, told Reuters.
Amid rising tensions over the past few weeks, more than 1,000 new refugees have fled to Bangladesh.
"My cousin in our village back in northern Maungdaw told me about rising tensions there. Military is everywhere. People are crying, mourning the dead," said Mohammed Shafi, who lives in a Rohingya refugee camp in Bangladesh near the Myanmar border.
"This morning we had a prayer meeting in the Kutupalong refugee camp for the victims and for the wellbeing of our homeland. Things are turning real bad. It's scary."