At least 77 Rohingya Muslims and 12 members of security forces were reportedly killed in the Rakhine state of Myanmar after a rebel group launched pre-dawn raids on police posts and tried to break into an army base.
The office of Myanmar's de facto leader, Aung San Suu Kyi, announced the death toll in a statement on Saturday, claiming the Rohingya killed were fighters.
A statement on Friday said that an estimated 150 fighters staged coordinated attacks at around 1am local time in the northern Maungdaw township.
The clashes came hours after a panel led by former UN chief Kofi Annan urged Myanmar to lift restrictions on movement and citizenship for Rohingya, a persecuted Muslim minority.
The Arakan Rohingya Salvation Army (ARSA) claimed responsibility for the attacks in a Twitter post but did not mention casualty figures or how many fighters were involved.
ARSA, accusing the Myanmar forces of killings and rape, said on Friday it was "taking defensive actions" in more than 25 different locations.
The township of Rathetaung in northern Rakhine has been under "a blockade for more than two weeks which is starving the Rohingya people to death", it said.
"As they prepare to do the same in Maungdaw … we had to eventually step up in order to drive the Burmese colonising forces away."
The group warned of more attacks to come.
ARSA was formed by Rohingya living in Saudi Arabia after a bout of serious communal violence in 2012, according to the International Crisis Group.
Suu Kyi's office said, "extremist Bengali insurgents attacked a police station in Maungdaw region in northern Rakhine state with a handmade bomb explosive and held coordinated attacks on several police posts".
Using the term "Bengali" is a derogatory way to describe the Rohingya Muslims, implying they are illegal immigrants from Bangladesh.
The fighters had seized weapons from police, Suu Kyi's office said.
Al Jazeera's Florence Looi, reporting from Yangon, said the violence would further divide Myanmar.
"It is going to affect sentiment on the ground. Already, the Rakhine Buddhists we spoke to don't want to live with the Muslim community. They say the Rohingyas side with terrorists or fighters.
"And on the part of the Rohingya villagers, we have received reports of beatings and indiscriminate killings, arbitrary arrests. So all of this makes the two sides more polarised."
The UN has expressed concern over Friday's violence, urging "all parties to refrain from violence, protect civilians and restore order".
The clashes mark an escalation in a conflict simmering in Rakhine since last October when similar events prompted a massive military operation that caused more than 80,000 Rohingya Muslims to flee to Bangladesh.
After a period of easing violence, tensions rose again in recent weeks with the military moving hundreds of troops into remote villages to flush out fighters amid a spate of killings of Buddhists.
The Rohingya Muslims are denied citizenship in Myanmar and are classified as illegal immigrants from Bangladesh, despite claiming roots in the region that go back centuries.
There are approximately 1.1 million Rohingya in Myanmar.
The mistreatment of the Rohingya Muslims, often described as the world's most persecuted minority, has emerged as Myanmar's most contentious human rights issue as it makes a transition from decades of military rule.
The UN believes Myanmar security forces may have committed crimes against humanity against the Rohingya Muslims. The military rejects the allegations.
Annan's Rakhine commission said Suu Kyi's government should respond to the crisis in a "calibrated" way without excessive force.
It warned of radicalisation on both sides if problems were not addressed quickly, advising Myanmar to address "legitimate concerns" of the Rohingya.
The commission was formed last year at Suu Kyi's request, and her government has previously vowed to abide by its findings.
Journalists and observers are denied access to northern Rakhine, and the government has refused entry to a UN mission seeking to investigate human rights abuses there.