South Sudan's warring rivals accused each other Wednesday of launching attacks, with the government army saying rebels had broken their own peace deal hours after signing.

The government refused to sign the power-sharing deal, despite the threat of international sanctions.

"There was a heavy attack by the rebels, but we fought back in self-defence and repulsed them," army spokesman Philip Aguer told AFP, describing battles Wednesday in the Manyo district of the northeastern Upper Nile state.

Reports of fighting could not be independently confirmed, but the trading of blame comes after a Monday deadline ended to sign a peace deal.

While rebel chief Riek Machar signed the document at talks in Ethiopia, President Salva Kiir only initialled part of it and said he would return to the table in early September to finalise the accord.

"You cannot sign a peace agreement and then launch an attack hours later," Auger said.

Rebels accuse the government of wanting a military solution to the 20-month war, and said they were attacked near the small but strategic southern town of Pageri on Tuesday, on the main road south from the capital Juba towards the Ugandan border.

Rebel general James Koang Chuol said his troops had seized Pageri, claiming they now controlled the key highway.

Aguer dismissed the rebel claims as "lies", saying that while two government soldiers were killed in fighting, they were attacked by unknown gunmen raiding cattle. Aguer said the government was in control of the town.

International frustration at the failure to strike a peace deal is at breaking point.

The United States and Britain on Tuesday pushed for UN sanctions against the government.

US National Security Advisor Susan Rice accused Kiir's government of a "failure of leadership" and said it had "squandered" another opportunity to end a conflict that has killed tens of thousands and which has plunged the world's youngest nation into chaos.

The government, who have called the proposed deal a "sellout", say it is not possible to sign a credible peace deal because rebel forces have split.

Powerful rebel general Peter Gadet and other key commanders earlier this month accused Machar of seeking power for himself, and said they would fight both their former comrade and the government.