Four civilians, including a woman and a child, were killed when two mortar bombs exploded inside a UN compound in the town of Melut in South Sudan's Upper Nile state, the UN reported on Wednesday.

Eight more people were injured in the blast on Tuesday evening, the UN mission in South Sudan said in a statement, adding that it is "deeply concerned by the disregard for civilian lives, UN personnel and facilities."

Fighting has recently intensified in South Sudan's Upper Nile and Unity states as government troops and rebel forces fight for control of territory. At stake are the Upper Nile oil fields of Paloch, which the rebels said they were poised to seize on Wednesday.