Ethio­pian leader Hailemariam Desalegn in 2012. He expressed his “deepest grief” over the attack in a televised address late Sunday. (Jenny Vaughan/AFP/Getty Images)

­Ethiopia’s leaders have vowed to hunt down gunmen from South Sudan who massacred about 200 villagers and kidnapped more than 100 children in a cross-

border raid last week, state media reported Monday.

Gunmen from the Murle tribe descended on a dozen villages in Ethiopia’s remote Gambella province early Friday, snatching children, shooting adults and carrying off more than 2,000 head of cattle.

“What we know is that they are heavily armed and well organized, and they knew what they were doing,” government spokesman Getachew Reda told The Washington Post. “Security forces have killed dozens of them, and some of them clearly were wearing military camouflage, but in that part of Africa, it is not entirely surprising.”

He added that Ethiopian forces are in discussions with South Sudan’s government and that a cross-border operation is a “distinct possibility.”

Although cattle rustling and violence between rival tribal groups is common in the remote region, the scale and ferocity of the attack was unprecedented.

In a televised address late Sunday, Prime Minister Hailemariam Desalegn expressed his “deepest grief” over the raid and said defense forces were pursuing the attackers and would free the children.

“The atrocities committed by armed militants from South Sudan’s Murle tribe claimed the lives of 208 mothers and children. They also abducted 102 children,” he said.

Ethiopians took to social media to express sorrow over the attack as well as outrage that official statements were not issued until days afterward.

Several top government officials, including the prime minister, were attending a high-profile African security conference in the city of Bahir Dar over the weekend.

Gambella province is more than 300 miles southwest of the capital and is lightly populated, but it is also home to nearly 280,000 refugees from the fighting in South Sudan, where a civil war raged until late last year.

The international community brokered a tense peace deal between the warring factions in South Sudan last year. On Monday, dissident Vice President Riek Machar, who has been in Ethi­o­pia, had been expected to fly from Gambella back to Juba, South Sudan’s capital, but his return was delayed at the last minute.

[South Sudan report shows civil war horror]

Huge swaths of South Sudan, which gained independence from Sudan in 2011, are out of the control of the central government. In the past two years, forces linked to the rival Dinka and Nuer tribes have carried out atrocities against each other.

Ethiopia has long feared that the violence would spill across the border. The main victims of Friday’s attack were Nuer tribesman living on the Ethiopian side of the border.

Gambella was also the scene of a bloody conflict this year between Nuer and Anuak tribesmen, during which local security forces were accused of siding with their respective tribes. The Ethio­pian government disarmed the regional police in response, which may have increased the locals’ vulnerability to cross-border raids.

Hallelujah Lulie, an analyst with the Institute for Security Studies in Addis Ababa, said cattle rustling and kidnapping have long been common in the region but never on such a scale.

“Some groups use abductions to weaken other groups, to adopt some of their children to look over the cattle or act as militiamen or be used as ransom,” he said. “It is the magnitude of the attack we are still trying to understand. We still don’t have the full picture.”

The attack comes at a sensitive time for Ethi­o­pia, which until last year had been heralded as an African success story because of its decade of high growth based on foreign investment and extensive infrastructure projects.

Last year, however, the rains failed. More than 10.2 million people, mostly in the north and east, now require food assistance. The government issued an appeal for $1.4 billion in international aid.

Civil unrest also has exploded across the Oromo ethnic region, which surrounds the capital. Rights groups estimate that 200 people have died in the past five months in clashes with security forces.

Read more:

History repeats itself in Ethiopia

Ethiopia confronts its worst ethnic violence in years

South Sudan thought it solved its child soldier problem. It hasn’t.

Today’s coverage from Post correspondents around the world