South Sudan, embroiled in border fighting with its northern neighbour Sudan for the past month, said on Friday that Sudanese-backed rebel militia had attacked a town in the South's oil-producing Upper Nile state, broadening the conflict.
"A militia that is supported by the Sudanese Armed Forces attacked a place near Malakal and the SPLA (South Sudanese army) has repulsed them," said SPLA spokesman Philip Aguer. "There are no details on casualties, they are still being pursued," he said.
Sudan's army spokesman, Al-Sawarmi Khalid, could not immediately be reached on his mobile phone. Khartoum denies supporting any rebels in South Sudan.
Khartoum and Juba accuse each other of supporting rebel militias to destabilise their opponents, and each denies the other's charges. South Sudan gained independence from Sudan in July after five decades of intermittent civil war.
Malakal is the administrative centre of Upper Nile, a volatile area bordering Sudan and Ethiopia. It is also a base for many UN agencies and international aid groups.
Fighting along the 1,800 km contested border in what was once Africa's largest country erupted in late March after Sudan and South Sudan failed to resolve a number of contentious issues including oil export fees and citizenship.
The skirmishes have threatened to escalate into a full-blown conflict, which neither can afford. Both economies have suffered from the shutdown of most of their oil production as a result of the conflict.
South Sudan seized the contested Heglig oilfield earlier this month, on which Sudan relied for about half its oil output, but withdrew after immense international pressure. Juba has since then accused Khartoum of launching air strikes on its territory, a charge Sudan denies.
China and the African Union have stepped up diplomatic efforts in the past week to try to bring the rivals back to the negotiating table.