JUBA (AFP) - South Sudan 's army said on Friday that a cross-border attack by Sudanese troops on a military base in oil-producing Upper Nile state left 18 fighters dead and 73 wounded. The Sudan Peoples Liberation Army (SPLA) said it expects more attacks, claiming Sudan intends to try and capture the south's oil fields. "There has been fighting in Kuek after an attack on our base by Sudanese armed forces (SAF) and mercenaries" that started at 9:00 am (0600 GMT) on Thursday and lasted seven hours, SPLA spokesman Philip Aguer told AFP. "On our side, five were killed and 26 were wounded and on the side of the attackers, 13 bodies were counted on the ground and they have 47 wounded," Aguer said, claiming they used mortars, AK-47 assault rifles and heavy machineguns. "We repulsed the attack, and the attackers were driven back to the Republic of Sudan , on the border with White Nile state," Aguer said. Sudan 's army spokesman was not immediately available for comment but emphatically denied on Thursday that the SAF was attacking camps or areas across the country's new international border with the south. Aguer said the SPLA expected more attacks in the Upper Nile area motivated by Khartoum's desire "to take the oil fields," echoing the words of President Salva Kiir, who claimed on Thursday that South Sudan 's former civil war enemy was planning to "capture the oil fields." Kiir said his northern counterpart Omar al-Bashirs accusations that Juba was funding rebel groups in Sudans southern states were "a prelude from Khartoum to justify their pending actions against South Sudan ." South Sudan seceded from the north on July 9 after a 22-year civil war and a January referendum in which the south voted almost unanimously to break away. On Thursday, a local official said 12 people had been killed and 20 wounded in air strikes on the Yida refugee camp in Unity state, the temporary home of around 20,000 people who have fled the violence over the border in South Kordofan. Unity and neighbouring Upper Nile are South Sudan 's only oil producing regions, and both border South Kordofan, where the SAF has been battling southern-aligned rebels since June. Aguer said he was not aware of any casualties from the six reported bombs dropped on Thursday and aid agencies on the ground have not spoken of any deaths or injuries. But security around the border regions and oilfields has been stepped up in anticipation of more attacks by the Sudanese army. "They have declared their intentions to unite the two Sudans by force," the SPLA spokesman said.