The vast territory has been wracked by violence since southerners voted in January to separate from the north and form a new nation. More than 1,500 people have since died in south-south violence, according to the United Nations.
At least seven rebel militias are at war with the government, the U.N. says. Gun battles between these militias and the army, and tribal warring over cattle, have erupted in nine of the south's 10 states.
South Sudan's internal crisis has been overshadowed by the threat of open conflict with the north, and persistent fighting in the border region in recent weeks has displaced tens of thousands and strained the fragile peace.
Analysts say that even if the north-south peace holds, the new country could still fail if it cannot rein in its internal violence.
"This was a cattle raid assumed to be carried out by armed militia from Unity state," said army spokesman Philip Aguer. "Seven police were killed including two officers, and 22 civilians." These were preliminary figures, he said, adding that the raid took place in Warrap state.
The southern army said it was holding defensive positions near the north-south border after it accused the north of twice bombing its territory, Friday and Monday.
The independence vote was the climax of a 2005 peace deal that ended a civil war which killed about 2 million people. North and south Sudan have fought for all but a few years since 1955, a conflict caused by ethnic, religious and ideological differences and oil.
The Republic of South Sudan is due to be founded on July 9.