The Carter Center has criticised the way in which the votes were recorded
The Carter Centre, which monitored the Sudanese elections last month, has questioned the accuracy of the results.
The vote count was "chaotic, non-transparent and vulnerable to electoral manipulation," said the group founded by former US President Jimmy Carter.
President Omar al-Bashir was declared the winner of Sudan's first multi-party elections in 24 years.
Observers and opposition parties complained of administrative problems, fraud and, in the south, intimidation.
The Carter Center urged Sudan's National Elections Commission (NEC) to publish the results in individual polling stations and to thoroughly review the results to increase transparency and public confidence.
Some observers had claimed a video posted on the video-sharing website YouTube, which apparently shows ballot stuffing by election officials, was proof of poll rigging.
The private Sudanese newspaper Al-Ra'y al-Amm reported on Monday that the NEC had launched an investigation on the basis of the video footage.
However, an NEC spokesman told the BBC that the people shown in the footage were not in fact working for the Commission and that the video was a fake.
The Carter Center report also criticised the way in which the votes were recorded.
"While alterations to results were often an attempt to correct mathematical errors, in some cases numbers were arbitrarily changed without clear explanation," it said.
Concerns were also raised over election-related violence which killed several people in South Darfur and Unity State.
President Bashir took 68% of the vote despite facing war crimes charges over Darfur and allegations of vote-rigging from opposition parties.
Meanwhile, former rebel leader Salva Kiir was re-elected in the semi-autonomous south, with 93% of the vote.