Greater openness from Khartoum about Sudanese oil revenues will be key to keeping the peace with the south after next week's independence vote, a campaign group warned on Thursday.

British-based organisation Global Witness said the Khartoum government had yet to give a satisfactory explanation for discrepancies between its own figures for crude output and those given by oil firms operating in south Sudan, notably Chinese giant CNPC.

It said the conflicting figures had already fanned southern mistrust of the Khartoum government during the interim period since a 2005 peace deal in which they were supposed to be sharing oil revenues generated in the south.

It warned those suspicions were likely to intensify if the region votes to break away in the week-long referendum which begins on Sunday.

"With both sides hugely reliant on oil revenues from the south, this issue is paramount going into the referendum," said Global Witness campaigner Rosie Sharpe.

"Suspicions over the sharing of oil revenues under the current peace deal have greatly added to the mistrust between the two parties, so the single best way to ensure stability after the referendum is to put a transparent and verifiable new oil deal in place," she said.

The group welcomed a commitment by the Khartoum government to allow an independent audit but said that it needed to be conducted promptly and its results made publicly available.

"The explanations provided by the Sudanese government and CNPC, the main oil company in Sudan, for why the company's oil production figures were significantly larger than those published by the government, do not stand up to scrutiny," its report, entitled Crude Calculations, found.

North and south are still negotiating how to share oil revenues if the referendum results in a vote for secession which would lead to southern independence in July.

Oil output from the south passes through a northern pipeline to the sea which could give the two sides an interest in reaching a mutually beneficial accommodation, some analysts argue.