The U.S. envoy to Sudan, Scott Gration, is holding talks with political leaders in Khartoum in an effort to save the credibility of this month's elections.
The talks began Thursday, a day after Yasir Arman, the presidential candidate of southern Sudan's dominant party, withdrew from the polls.
Arman and officials from the former rebel Sudan People's Liberation Movement said the move is necessary because of electoral irregularities and continued conflict in Darfur.
The SPLM is part of a fragile coalition government led by the National Congress Party of President Omar al-Bashir. Arman was seen as one of the favorites to challenge Mr. Bashir in voting that begins April 11.
The SPLM says it will still contest regional and legislative elections across Sudan, but not in Darfur.
In a joint statement Wednesday, the United States, Britain and Norway voiced concern about restrictions on political freedoms in Sudan, and called for a credible and peaceful vote to take place.
On Tuesday, the International Crisis Group accused Sudan's ruling National Congress Party of trying to rig the elections.
The conflict prevention group says the NCP has drafted election laws in its favor. It also says President Bashir's party has "bought tribal loyalties" and used flawed census results to draft electoral districts.
The Brussels-based group was especially critical of the electoral process in Darfur. It said the ruling party went to great lengths to count its supporters in Darfur in the 2008 census but made little effort to include the nearly 2.6 million people who have been internally displaced by the region's ongoing conflict.
The Darfuri vote is critical because nearly 20 percent of Sudan's citizens live in the region. Darfur also holds 86 of the 450 seats in the national assembly.
The vote will be Sudan's first multi-party elections since 1986 and it is a key part of a 2005 peace deal that ended the country's north-south civil war.
On Monday, President Bashir said if former southern rebels boycott the elections, the Sudanese government will not allow the south to hold a planned referendum on becoming independent.
Some information for this report was provided by AFP, AP and Reuters.