The state earlier was the site of a brutal counterinsurgency campaign by the military against the Muslim Rohingya minority, causing more than 700,000 to flee to neighboring Bangladesh.
Military officers said at a news conference in the capital, Naypyitaw, that leader Aung San Suu Kyi ordered security forces to launch the offensive against the Arakan Army.
The insurgent group, which seeks autonomy from the central government, killed 13 police officers and wounded nine in attacks on Jan. 4. The moves to counter the rebels were decided at a Jan. 7 meeting at Myanmar's presidential offices, the officers said.
Suu Kyi "said the Arakan Army is just a terrorist group and instructed us to defeat them effectively, quickly and clearly," Maj. Gen. Nyi Nyi Tun, vice chairman of the Myanmar Military Information Committee, told reporters. A terrorist designation criminalizes a group and bans all communication with them.
He said Suu Kyi suggested that if she did not order the military to attack the Arakan Army, the international community would accuse her of religious prejudice for attacking the Muslim guerrillas of the Arakan Rohingya Salvation Army but not Buddhist rebels who committed similar actions with similar goals.
The military in Buddhist-majority Myanmar has been accused of ethnic cleansing, and even genocide, against the Muslim Rohingya. Its counterinsurgency campaign was triggered when a group of Rohingya guerrillas attacked security outposts in August 2017.
The officers said the military clashed with the Arakan Army 15 times in 2015, 26 times in 2016, 56 times in 2017 and 61 times in 2018, while the rebels also planted some mines. They said there have been at least eight armed encounters this year. The guerrillas are known to have trained in areas controlled by other ethnic rebel forces, especially in Kachin state.
The Arakan Army, founded in 2009, is estimated to have several thousand well-armed and organized uniformed members, in contrast to the ragtag and virtually dormant Rohingya guerrillas.
Myanmar's military in December announced cease-fires in five areas where ethnic rebellions are active, but did not include Rakhine state because it had information that the Arakan Rohingya Salvation Army planned attacks, the officers said.
Brig. Gen. Zaw Min Tun said he believed that the fighting with the Arakan Army would not interfere with plans to repatriate Rohingya refugees from Bangladesh.