JUBA, Sudan (AFP) - Gunmen in south Sudan killed 12 people, including 10 soldiers, in an ambush on an army truck in oil-rich Unity state, a southern army spokesman said on Thursday. Attackers ambushed a truck an estimated 25 kilometres (15 miles) northwest of the state capital Bentiu on Wednesday morning, said Philip Aguer, spokesman for the ex-rebel Sudan Peoples Liberation Army (SPLA). Ten of our soldiers were killed, as well as two civilians in a planned attack, he said. Aguer accused the gunmen of being backed by the northern army, the Sudan Armed Forces (SAF). We suspect that this was an attack by an SAF militia, Aguer said, but could not provide further details on the claim. We are not taking any retaliation but just monitoring the border areas. Unity state lies along the tense border between north and south, and is one of Sudans main oil-producing areas. The situation is now calm but people are not happy about this, of course, he added. Last week the southern army accused the north of wounding four soldiers and two civilians in a helicopter gunship attack, a claim the north denied. The reported attack followed an earlier bombing raid on the south that the north did admit, but which it claimed targeted Darfuri rebels, not southerners. South Sudan is still recovering from decades of war with the north, during which about two million people were killed in a conflict fuelled by religion, ethnicity, ideology and resources, including oil. Tensions are rising as the south prepares to vote on January 9 in a referendum on potential full independence. Most analysts expect the south to vote to break away and split Africas largest nation in two.