However, some witnesses say the latest attack was aimed at the security forces.

'It seems that the focus of the attack was on the organised forces, however we can't confirm the accuracy of this information,' Zerrouk said.

Many in Southern Sudan believe that Khartoum is orchestrating the violence in order to undermine Southern Sudan's progression towards full independence.

A fragile peace has held between north and south since the end of a 21-year civil war in 2005.

The peace deal that ended the war laid out a road map that included elections in Southern Sudan, to be held next year, and a referendum on independence in 2011.

Southern Sudan's ruling party and Sudanese President Omar al-Bashir's National Congress Party are expected to contest the elections.

However, the UN is warning that if the violence continues, it will be difficult to organise the crucial ballot in the south.