Rebel forces in South Sudan accuse the government of breaking a cease-fire the two sides signed Thursday in Ethiopia.
A statement from a pro-rebel general Friday says forces of President Salva Kiir, along with an armed group from Sudan's Darfur region, have attacked rebel positions at Duar in South Sudan's Unity state.
The statement from Brig. Gen. Lul Ruai Koang also says Kiir's forces and Ugandan soldiers attacked rebel positions south of Bor, the capital of Jonglei state.
There has been no independent confirmation of the report.
Government and rebel delegates signed a ceasefire agreement Thursday in Addis Ababa after three weeks of talks mediated by the Intergovernmental Authority on Development (IGAD).
At the ceremony, speakers warned the agreement may be hard to implement.
Analyst Peter Pham of the Atlantic Council told VOA there are questions about whether the government and rebels have full control of their forces.
"On neither side are we dealing with professional militaries of the sort that Americans and Europeans or East Asians are used to," Pham said. "These are units that answer to particular generals, some of whom are self-appointed, even."
U.N. humanitarian chief Valerie Amos begins a three-day trip to South Sudan Monday. The U.N. said she will meet with government officials and aid groups in an effort to draw attention to the "humanitarian consequences" of the country's unrest.
The unrest began in mid-December after Kiir accused his former vice president Riek Machar of attempting a coup, a charge Machar has denied.
U.N. Assistant Secretary-General for Human Rights Ivan Simonovic says the fighting has killed thousands of people. An estimated 500,000 have been displaced from their homes.
The U.N. World Food Program on Friday reported the looting of a warehouse in Malakal, the capital of Upper Nile state, where heavy clashes were reported last week.
The cease-fire signed Thursday calls for an immediate cessation of all military operations. Another agreement deals with the status of 11 political opposition supporters the government arrested after hostilities began in mid-December.
The government's refusal to release the detainees was a sticking point in the peace talks.
The government said on its Twitter account Thursday that the detainees will be given amnesty after they are taken to court.