More people have died in southern Sudan than Darfur this year
At least 25 villagers have been killed in ethnic militia raids in the semi-autonomous region of southern Sudan.
The southern military is blaming the unrest in oil-rich Upper Nile State on gunmen backed by a new splinter group from the south's ruling party.
But the SPLM-DC leader has rejected the claims and denies he has a militia.
It is the latest in a string of bloody clashes observers fear threaten a 2005 peace deal which ended the 21-year civil war between the north and south.
The BBC's Peter Martell in the southern capital, Juba, says the south has claimed the attacks are being deliberately encouraged to destabilise the region ahead of elections in April and a referendum on potential full independence for the south in 2011.
Fighting is common in the region, often over cattle or land, but the scale of recent violence - separate from the conflict in Darfur - has left many in shock, he says.
Our reporter says militia fighters attacked the village of Bony-Thiang about 30km (19 miles) north of Malakal, the capital of Upper Nile State, early on Friday morning.
They killed 20 people including the chief, his wives and three children, and wounded many more.
The gunmen, who came from the Shilluk ethnic group, also burnt the village belonging to the Dinka people.
Dinka men then retaliated by attacking a Shilluk village, killing five residents, including three children.
The southern army says it rescued a wounded and abandoned two-year-old child from the village.
Correspondents say the incident rapidly acquired a political dimension.
But Lam Akol, a former foreign minister who formed the SPLM-DC three months ago, has denied accusations that he is linked to the attacks by backing the Shilluk militia.
Fighting is common in the region, often over cattle or land
His party has taken some supporters from the Sudan People's Liberation Movement (SPLM), the historic party in the south.
Observers say the war of words shows the SPLM's discomfort with the newcomer.
According to the UN, more than 2,000 people have died and over 250,000 been displaced in inter-tribal violence across southern Sudan since January.
It warns that the rate of violent deaths now surpasses those in Darfur.