The US has urged Sudan to eliminate restrictions on political parties in the run-up to general and presidential elections.
Philip Crowley, the US state department spokesman, said on Monday that the US had concerns over "serious restrictions" on political freedom.
"[It is] important for the government of Sudan immediately to lift restrictions on political parties and the civil society," Crowley said.
"We will judge these elections on whether they reflect the will of the Sudanese people, whether they meet international standards. We currently see disturbing trends in both areas."
Sudan is to hold the elections from April 11 to 13 - its first national poll since 1986.
The electoral commission has said that the poll will take place despite the threat by opposition parties to stage a boycott.
The government has been accused of adopting repressive security measures and providing unequal access to state media by the opposition Umma party, who have also said that public finance and commitment to Darfuri representation in the presidency should be ensured.
Umma and several other opposition parties called on Friday for the government to implement crucial reforms within four days.
They said that they would not participate in the vote, which should be delayed until May if the reforms were not made.
Mariam al-Sadiq al-Mahdi, a senior member of Umma, told Al Jazeera on Monday that the national census on which voter registration is based was "controversial".
"We demanded a revision [of the census], but it did not take place. That is also applies to voter registration," she said.
"Therefore, we decided to go further if we do not succeed in turning the elections [into] real democratic transformation, and with genuine free and fair elections.
"We asked to postpone the elections to November. Then we reduced it to four weeks only. We will not participate in the presidential elections if our demands are not met.
"They want to have the election to re-elect the president to give him something to face his problems with the International Criminal Court (ICC)."
The ICC, based in The Hague, Netherlands, has charged Omar al-Bashir, the incumbent president, with war crimes for alleged attrocities in the conflict in the western Darfur region.
The uncertainty over the holding of elections coincides with frustration in diplomatic quarters over the lack of progress on the conflict in Darfur.
Scott Gration, the US special envoy to Sudan, held a meeting with Ghazi Salah al-Din, an adviser to al-Bashir, in Khartoum on Monday to discuss the results of peace talks between a number of opposition factions.
|Gration says peace negotiations with Darfuri groups have made little progress Reuters]
Gration signalled that the negotiations had made little progress.
The developments came as the Justice and Equality Movement (Jem), one of the groups involved in these talks, accused the Khartoum government of carrying out attacks on its positions in northern and western Darfur since Sunday and of violating a ceasefire deal signed in February.
Ahmed Hussein Adam, the Jem spokesman in the Qatari capital Doha for the peace talks, said that 10 civilians had been wounded.
The Sudanese army has denied the accusation.
Fighting between opposition groups in Darfur and the government began in 2003 and has killed about 300,000 people.
Sudan also suffered two civil wars between the north and south of the country over the past five decades.