Addis Ababa: South Sudan`s rebels on Saturday accused government forces of attacking their positions, just hours after the two sides promised an immediate halt to their nearly 11-month-old civil war.
Chief rebel negotiator Taban Deng said troops loyal to President Salva Kiir launched an assault earlier in the day from their positions around the hotly-contested northern oil hub of Bentiu.
He said the rebels "condemn in the strongest terms possible the continuous aggression and violation" of the ceasefire, and called on regional peace mediators to investigate the incident.
South Sudan`s army spokesman Philip Aguer, however, insisted government troops "have not moved anywhere" and that he was unaware of any fresh outbreaks of fighting.
Earlier Saturday, Kiir and rebel leader Rief Machar ended two days of fresh talks in the Ethiopian capital Addis Ababa, and pledged to an "unconditional, complete and immediate end to all hostilities."
The East African regional bloc IGAD, which has been trying to broker a peace deal, has given the pair just 15 days to finalise a transitional power-sharing accord.
Kiir and Machar signed a ceasefire at the start of the year and several subsequent deals to renew it, but the truces have been short lived. IGAD said yet another violation would have severe consequences.
"Any violation of the cessation of the hostilities by any party will invite... collective action by the IGAD region," a statement said, listing asset freezes, travel bans and an arms embargo as possible sanctions.
"The IGAD region shall, without further reference to the warring parties, take the necessary measures to directly intervene in South Sudan to protect life and restore peace," the statement added.
The civil war has left tens of thousands dead, forced almost two million from their homes, pushed the country to the brink of famine and has been marked by widespread atrocities by both sides.
Fighting started in December last year, when Kiir accused his sacked deputy Machar of trying to stage a coup, with the violence broadening into an ethnic conflict and now including more than 20 different armed groups.Regional leaders have grown increasingly impatient with the warring sides, their slow-moving talks and repeated ceasefire violations, and prior to the latest meeting bluntly told Kiir and Machar to "come to their senses".
Recent weeks have seen an upsurge in fighting coinciding with the end of the rainy season, and there have been heavy clashes in several areas.
Speaking after the talks, Kiir vowed that his troops would only fight in self-defence -- although this has been used as an excuse by both sides for previous truce violations.
"From this hour, they won`t be found outside their barracks to make an attack from any direction. They should fight only in self defence," he said.
Machar also said he had ordered all rebel fighters "to cease the hostilities and remain in their locations and only act in self-defence".
"We do not want any soldier or any civilian to die again after this progress in Addis Ababa," he said earlier Saturday.
IGAD, an East Africa and Horn of Africa trade bloc, groups Djibouti, Ethiopia, Kenya, Somalia, South Sudan, Sudan and Uganda. The bloc has been trying to broker peace in South Sudan since January.
South Sudan gained independence from Khartoum in 2011 and is the world`s youngest nation.