The UN Security Council is "dead serious" about a threat to impose sanctions on Sudan and South Sudan if they do not complete a peace deal, US ambassador Susan Rice said Thursday.

The Security Council and the African Union "intend to hold the parties' feet to the fire" to keep their promises, Rice told reporters after a meeting on disputes between the neighbors who came close to all out war this year.

AU envoy Thabo Mbeki, the former South African president, briefed the council by video-conference on his efforts to ease tensions between Sudan and South Sudan, who separated in July last year.

The UN Security Council had given the rivals until August 2 to resolve all their disputes. But it was only a day later that they struck a first accord on how to share the wealth from oil reserves that straddle the unsettled border.

Setting a border, control of the disputed territory of Abyei, citizenship and other disagreements remain. Their armies have clashed repeatedly this year.

Rice said the oil accord was encouraging but that it was "vitally important" to carry out all parts of UN resolution 2046 that set the August 2 deadline. "It is not a Chinese menu from which the parties can pick and chose," she commented.

Sudan and South Sudan "must understand that the African Union and the United Nations Security Council are dead serious about implementation of their decisions and resolutions," said Rice.

"This council has been very clear in underscoring its intention to consider additional measures," including economic sanctions, if their accords are not fully implemented.

The envoy said the Security Council was not setting a new deadline for an overall accord between Sudan and South Sudan, which fought a two decade civil war up to 2005 in which two million people died.

She said UN Secretary General Ban Ki-moon had to produce a report on the Sudan rivalry by September 2 and Mbeki would brief the council again.

The Security Council wants the border, Abyei and other disputes resolved "in very short order and we are ready to respond to their actions," Rice said.

"Nobody is expecting this to draw itself out over a period of months without a response from the council, quite the contrary, but we are also realistic," she added. "We are very clear that the two parties have to move quickly."

Rice also welcomed Sudan's agreement to let humanitarian agencies into its conflict-stricken South Kordofan and Blue Nile states but said it was also urgent for this to be carried out. The United Nations says there is a humanitarian crisis in the two states.