A series of attacks targeting border posts along Myanmar's border with Bangladesh have left nine police officers dead, officials in Rakhine region have told the BBC.
The attacks on three police posts near Maungdaw early on Sunday appeared to be co-ordinated, they said.
Police said those responsible were from the persecuted Rohingya minority group.
Rakhine has seen simmering tension between its Buddhist and Rohingya Muslim populations.
Several attackers were also killed in Sunday's attacks, an official in Maungdaw told BBC Burmese.
The assailants looted more than 50 guns and thousands of bullets from the guard posts, police said.
They were said to be armed mainly with knives and home-made slingshots that fire iron bolts.
Communal violence in Rakhine state in 2012 left scores dead and displaced more than 100,000.
Those who attacked the border posts had shouted that they were Rohingyas, police general Zaw Win told reporters at a press conference in the capital, Nay Pyi Taw.
He did not explain their motivations or name a specific group.
Senior Rakhine state government official Tin Maung Swe earlier told the AFP news agency that he blamed the Rohingya Solidarity Organisation (RSO), a small militant group that was active in the 1980s and 1990s, for the attacks.
The RSO has been blamed by the government for other attacks in recent years but observers believe it has been dormant for some time.
Former UN Secretary-General Kofi Annan is heading an advisory commission looking into sectarian divisions in Rakhine state.
Buddhist nationalists in Myanmar see the country's estimated one-million Rohingya as Bangladeshi intruders, despite many having lived in the country for generations.
The government of Myanmar - previously known as Burma - refuses to grant them citizenship.