Rebel forces in South Sudan are accusing the government of violating the cease-fire the two sides signed Thursday in Ethiopia.
A pro-rebel general said Friday that forces of President Salva Kiir have attacked rebel positions in Unity and Jonglei states.
General Lul Ruai Koang said government troops were aided by JEM rebels from Sudan's Darfur region in the Unity attacks, and by Ugandan forces in Jonglei.
South Sudan army spokesman Philip Aguer said on Twitter that he "received no reports of fighting" ahead of the cease-fire, which is due to take effect Friday evening South Sudan time.
Government and rebel delegates signed the cease-fire agreement in Addis Ababa after three weeks of talks mediated by the East African regional bloc 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. These are units that answer to particular generals, some of whom are self-appointed, even."
U.N. humanitarian chief Valerie Amos will begin 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 President 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.