Armed men from South Sudan have killed around 140 people and kidnapped a number of others in a cross-border raid into Ethiopia, the Ethiopian government said Sunday.
Ethnic Murle gunmen on Friday "attacked near Gambella and killed close to 140 people. They also abducted some of them," Ethiopian foreign ministry spokesman Tewolde Muluteg told AFP.
The Murle, a tribe from South Sudan based in the eastern Jonglei region, often stage raids to steal cattle.
They attacked the Nuer tribe, one of the two main ethnic groups in South Sudan, but who also live across the border in Ethiopia.
The western Ethiopian region of Gambella, which borders South Sudan, is also home to some 272,000 South Sudanese refugees who have fled the civil war that erupted in their country in December 2013.
"Our forces have been in pursuit of the attackers and they decimated scores of them," Muluteg said, without indicating whether the Ethiopian forces entered South Sudan territory.
"In border areas cattle feuds and raids are not uncommon. Of course, something of this magnitude is different," he added.
"We don't think (the armed men) have any links to the South Sudan government or the rebels."
Ethiopia has been heavily involved in the South Sudan peace process, partly because of the risk that the conflict could destabilise Gambella.
South Sudan rebel leader Riek Machar is due to return to South Sudan's capital Juba on Monday from his rebel base at Pagak in the far east of the country, close to the Ethiopian border, rebel spokesman Colonel Nyarji Roman said.
Machar, who has not set foot in Juba for two years, is to form a transitional government with his rival, President Salva Kiir, as part of a peace deal signed in August.
Machar, who was Kiir's deputy before the war, has been living in exile in Kenya and Ethiopia, but was re-appointed vice president in February.
He is expected to be swiftly sworn into office as vice president at the presidential palace alongside Kiir on Monday but a welcome rally by his supporters may be cancelled amid government security fears.
After winning independence from Sudan in 2011, South Sudan descended into war two years later, setting off a cycle of retaliatory killings that have split the poverty-stricken country along ethnic lines.
Tens of thousands have been killed and over two million people forced to flee their homes during the war.
Both the government and rebel sides have been accused of perpetrating ethnic massacres, recruiting and killing children and carrying out widespread rape, torture and forced displacement of populations to "cleanse" areas of their opponents.