More than 1,000 gunmen from South Sudan have killed 28 people and kidnapped 43 children in neighbouring Ethiopia, according to a government official.

Chol Chany, a regional government spokesman, said on Wednesday the raids occurred on Sunday and Monday in Gambella region's Gog and Jor areas, which border South Sudan's Boma region.

"Murle bandits carried out the attack. They fled along with 43 children," Chany told Reuters news agency, using a term for a local ethnic group.

"The [Ethiopian military] is pursuing them. The assailants haven't crossed over to South Sudan yet."

According to AP news agency, Mawien Makol Arik, spokesman for South Sudan's foreign ministry, was aware of fighting in Gambella.

He said Ethiopian troops have not crossed the border into South Sudan in pursuit of the accused attackers.

Al Jazeera's Catherine Soi, reporting from Kenya's Nairobi, said it was a challenge to get information from Gambella because it was a very remote area and telephone network was "very patchy".

"It is not really a surprise that we are hearing about the incident days later," she said.

She also said that such cross-border attacks were common, but the magnitude of the latest raids were raising concerns in Ethiopia.

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The latest raids took place almost a year after similar attacks in the province's Jikawo and Lare areas, which border South Sudan's Upper Nile State.

Then, more than 200 people died and about 160 children were kidnapped.

About 100 children have managed to return to Ethiopia, but the rest remain in the kidnappers' hands, Chany said.

Civil war

Oil-rich South Sudan has been mired in a civil war since President Salva Kiir, an ethnic Dinka, fired his deputy Riek Machar, a Nuer, in December 2013.

The resulting conflict has split the country along largely ethnic lines and forced more than three million people to flee their homes.

More than one million of them have found refuge in neighbouring countries, especially Ethiopia, Uganda, Kenya and Sudan.

Gambella alone is currently hosting up to 300,000 South Sudanese refugees.

Regional governments have expressed fears that violence in South Sudan could spill over its borders into their own nations.