An international human-rights group says widespread abuses in Sudan's April election must be addressed now so the same problems are not repeated in the referendum on self-determination in southern Sudan.

Human Rights Watch senior researcher Tiseke Kasambala discussed the group's report on Sudan's elections.

"We are calling for decisive action and immediate action to end the impunity that has marred Sudan's history for the past 20 years, to ensure that there is no impunity for abuses that took place during the elections, so that there is a peaceful and meaningful referendum in 2011," said Kasambala.

Human Rights Watch cites a number of crucial problems with the April election, including restrictions on free speech and the use of intimidation and arbitrary arrest by Sudanese security forces.

The election came as a result of the 2005 Comprehensive Peace Agreement, which ended a civil war that spanned two decades. The agreement also paved the way for a referendum on whether south Sudan will secede from the north.

The vote is set to take place in January 2011.

Kasambala says Sudanese and international authorities ignored violations in the April election for the sake of the referendum.

"Well we think that some of the political considerations around the referendum have made the international community, in particular donors like the U.S. government and the European Union, to hold back from actually trying to criticize what took place during the elections," said Kasambala. "But in our view that is actually going to have the opposite effect, because it is basically sweeping all the problems under the rug, not addressing them properly, and that does not bode well for the referendum."

Kasambala says Sudan's national unity government needs to enact reforms that were called for in the peace agreement, including reform to the national security apparatus.

Analyst Roger Middleton is from the London-based research group Chatham House. He says the referendum is the most crucial event in Sudan's peace process.

"Many people looking at Sudan have said that actually the referendum is the event that has the most potential for causing violence and a return to war," said Middleton. "And I think some people may have accepted the election result as something which allows the referendum to happen and if that is able to happen peacefully, perhaps that is something that some analysts might say is an acceptable sacrifice."

He says there is a great raft of issues that need to be dealt with before the referendum takes place. Education centers need to be set up on how to run polling stations and count votes, security needs to be put in place to keep the population safe during the election, and a proper appeals process needs to be organized to deal with complaints. Without this infrastructure, he says, the referendum may have a violent backlash.

"If the referendum is not seen to be fair, if people feel they have not been able to express themselves through the ballot box in that referendum, there is a real danger of a return to violence, I think," he said.

The African Union said Sudan's April election was "free and fair", considering the challenges faced in the country.

International observers from the European Union and the Carter Center said the elections failed to meet international standards because of widespread problems.