YANGON, Myanmar (AP) — An attack by ethnic Rohingya militants in western Myanmar left 12 security personnel and 77 Rohingya Muslims dead in a dramatic escalation of communal violence that has plagued the region, as the United States urged authorities to avoid a response that would inflame the tensions.

The office of the country’s leader, Aung San Suu Kyi, said Friday that military and border police responded to the attacks by launching “clearance operations.”

Police fought off groups of as many as 100 Rohingya attackers armed with guns, machetes and homemade grenades. The captured weapons were shown in photos posted online by the government.

A witness in Maungdaw township in Rakhine state, contacted by phone, said soldiers entered her village at about 10 a.m. Friday, burned homes and property, and shot dead at least 10 people.

The witness, who asked to be identified by her nickname, Emmar, because of fear of retribution, said villagers fled in many directions but mostly to a nearby mountain range. She said gunshots and explosions could be heard and smoke could still be seen Friday evening.

A militant group, the Arakan Rohingya Salvation Army, or ARSA, took responsibility for the Thursday night attacks on more than 25 locations, saying they were in defense of Rohingya communities that had been brutalized by government forces. It issued its statement on Twitter on an account deemed legitimate by advocates of Rohingya rights.

Suu Kyi called the attacks “a calculated attempt to undermine the efforts of those seeking to build peace and harmony in Rakhine state.”

State Department spokeswoman Heather Nauert said in Washington that as security forces act to prevent further violence and bring the perpetrators to justice, they should respect the rule of law and protect human rights and fundamental freedoms.

She said the attacks underscored the importance of the government implementing recommendations of a commission chaired by former U.N. chief Kofi Annan, which published its final report on Thursday recommending that the government act quickly to improve economic development and social justice in Rakhine state to resolve violence between Buddhists and the Rohingya.

Suu Kyi’s office said on its Facebook page that the attacks were intended to coincide with the release of Annan’s report.

The clashes were deadlier than an attack by the militants on three border posts last October that killed nine policemen and set off months of brutal counterinsurgency operations by Myanmar security forces against Rohingya communities in Rakhine state. Human rights groups accused the army of carrying out massive human rights abuses including killing, rape and burning down more than 1,000 homes and other buildings.