“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.

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 destabilize 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 center 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.