The decision not to commit to full withdrawal comes amid concerns the party's grassroots activists may not support extending the boycott.
"Their lower ranks have invested time and personal money in their campaigns," one source said. "They may see a revolt if they go for a full boycott."
The party's other demands include freezing oppressive security laws, creation of a body to oversee the National Elections Commission they accuse of bias towards the NCP, fair access to state media, and for the ruling party to stop using state resources in its campaign.
"If these eight conditions are not fulfilled by April 6, the Umma party will boycott all the process of elections," Sara Luqdallah, a senior party official, said.
Meanwhile, the Democratic Unionist Party (DUP) appeared to retreat from an earlier committment to join a full boycott following talks with opposition leaders and the NCP on Friday.
Amid continuing uncertainty over the elections, Scott Gration, the US envoy, arrived in Khartoum on Thursday. Umma party officials said Gration was "trying to achieve the delay" to salvage the elections' credibility.
Analysts said that this week's withdrawals from the presidential election, triggered by a shock annoucement from the Sudan People's Liberation Movement (SPLM) on Wednesday that its presidential cadidate would not run, had effectively handed a hollow first-round victory to Omar al-Bashir, the sitting president.
The boycott comes after al-Bashir repeatedly dismissed calls from opposition parties for the elections to be delayed until November on the basis that the country is not ready to go to the polls.
The opposition says many candidates have not been given fair opportunity to carry out significant electoral campaigns in the volatile country.
They have insisted that going ahead with the elections would be a "disaster" for Sudan.
Speaking on Wednesday, Khalil Ibrahim, the leader of the Justice and Equality Movement (Jem), the biggest opposition group in Darfur, told Al Jazeera that many eligable voters are being excluded from taking part in the polls.
"These elections are based mainly on false senses, especially in Darfur," he said. "Masses of populations ... will be excluded from the elections."
'Chaos and war'
Ibrahim warned that an al-Bashir victory would be a catastrophe for the country. "He will continue the violence, especially in the west part of Sudan," he said.
"I don't think the other parties will accept this, there will be chaos and war if he [al-Bashir] wins."
But al-Bashir's NCP deny that there is a popular appetite for delaying the polls and say they will push ahead with the process.
Dr Omar Rahman, a senior NCP member, told Al Jazeera that people in Sudan are keen for the elections to go ahead.
"Whether they [the SPLM] participate or not, the people have decided that elections are something they have looked forward to for years," he said on Thursday, before news of the opposition boycott broke.
"We are looking forward to peaceful and transparant elections."
The NCP has ruled Sudan in a coalition with the SPLM since a peace deal ended a 22-year civil war between North and South Sudan in 2005, and many see successful elections as a key part of a fragile peace process.
The north-south war claimed an estimated two million lives and destabilised much of East Africa.