Southern Sudan's main political party withdrew its candidate from the country's upcoming presidential election and said it will not contest parliamentary and local polls in Darfur, a move that erodes the credibility of Sudan's first multiparty election in decades.

The surprise announcement from the Sudan People's Liberation Movement comes as more than a dozen northern opposition parties and independent candidates also consider a boycott of the April 11 vote amid concerns that alleged government control of the media and biased legislation make a fair vote impossible.

The decision was made as the U.S. envoy to Sudan, Scott Gration, arrived in Khartoum late Wednesday. Gration was holding talks Thursday with political leaders in Khartoum about the upcoming vote, US Embassy spokeswoman Judith Ravin said.

SPLM party spokesman Yien Matthew said party officials plan to meet with Gration for talks Thursday aimed at warding off a crisis 10 days ahead of the country's first multiparty elections in 26 years.

Matthew said the party decided late Wednesday to withdraw its presidential candidate, Yassir Arman, from the national race. He added that the party also will not contest parliamentary polls in the volatile region of Darfur, but will participate in parliamentary and local elections in the rest of Sudan.

The party's decision stemmed from "numerous violations" committed by President Omar al-Bashir's ruling National Congress Party and "instability and state of emergency in Darfur," Matthew said.

"The SPLM can't put itself in fire with its people and supporters for participating in flawed elections," he said. "We will not reconsider our decision and we may develop it further."

He said the party is in talks with other opposition groups and warned they could decide in unison for a total election boycott within days.

The SPLM is the junior partner in al-Bashir's governing coalition, and the party's boycott throws its relations with the president's party into jeopardy. However, Arman was considered al-Bashir's most serious challenger in the presidential race.

The national and presidential elections are a crucial step in the 2005 north-south peace deal that ended a 21-year civil war and paves the way for a referendum when southerners would decide whether they will opt for secession from the Muslim-dominated north.

Some 2 million people died during the war. It is separate from the Darfur conflict which erupted in 2003 and has left 300,000 people dead. No comprehensive peace deal has been reached for Darfur.

Al-Bashir has already obtusely snubbed his southern partner, warning that the much-coveted 2011 referendum for southern Sudan is in danger if opposition parties press for postponing the elections.

The opposition has accused the president's National Congress Party of using state resources, limiting their access to the media and controlling the independent National election commission, undermining their chances and the fairness of the process.

International observers and rights groups have said all signs point to a flawed process where the National Election Commission is unlikely to deliver a free and fair process and on time.

On Wednesday the United States, Norway and Britain expressed worry about reports of restrictions on political freedoms, saying in a statement that all parties must make sure peaceful and credible elections are held in April.