The chief African Union (AU) mediator at peace talks for Sudan's war-torn Darfur region has expressed frustration at the stance taken by the rebels.
The rebels want changes to the post-conflict security arrangements
The warring parties have been given an extra 48 hours to reach a deal after missing Sunday's midnight deadline.
The Sudanese government has agreed to sign the AU-backed agreement, but the rebels have no common position.
The UN mission in Sudan says life for Darfur's displaced people will get much worse if a deal is not signed soon.
The BBC's Alex Last, reporting from the peace talks in Nigeria's capital, says so far there has been no breakthrough and the prospects do not look good.
US Deputy Secretary of State Robert Zoellick will travel to Abuja in a bid to hammer out an agreement before Tuesday's midnight deadline, AFP news agency reports.
The three-year conflict has killed some 200,000 people and driven more than 2m from their homes.
Chief AU mediator Salim Ahmed Salim told the BBC the rebels had come to the talks in Abuja with their minds made up, and had simply been repeating their demands.
DARFUR DRAFT PEACE PLAN
Pro-government Janjaweed militia to be disarmed
Rebel fighters to be incorporated into army
One-off transfer of $300m to Darfur
$200m a year for the region thereafter
The rebels want one of Sudan's vice-presidents to come from Darfur.
This was agreed for the south, under an agreement last year to end 20 years of conflict there.
"We've discussed this extensively, and it's not possible because the Sudanese constitution makes it clear that during the transitional period there will be two vice-presidents," Mr Salim said.
He told all the parties that the world would not forgive them if they failed to reach an agreement.
The two rebel groups are still seeking a number of changes, particularly over the post-conflict security arrangements such as the number of their forces to be integrated into the Sudanese army.
The AU mediators hope if they can get concessions on this issue from both sides the rest will fall into place.
But Khartoum says it has conceded enough.
"They [the rebels] will have to be reasonable and they have to pay also a price for this peace," Sudan's Justice Minister Muhammad Ali al-Maradi told the BBC.
The vice-president of Sudan has now flown home, after accepting the deal on Sunday, which is not a good sign, our correspondent says
Meanwhile, the 7,000 AU peacekeepers in Darfur are struggling to stop the violence between the rebels and pro-government Janjaweed militias.
Hundreds of thousands have been displaced by the violence
In the last 10 days, rival rebel factions have clashed while government forces and militia are closing in on the rebel town of Gareda.
With more than 100,000 people already sheltering in Gareda, any offensive could trigger a fresh humanitarian disaster, the UN says.
Sudan's government and the pro-government Janjaweed militias are accused of war crimes against the region's black African population, which the US has labelled as genocide.
But Mr al-Maradi dismissed such claims as "sheer propaganda".
Over the last three years the conflict, which began when rebel groups began attacking government targets, has become a regional crisis drawing in Chad, Eritrea and the Central African Republic.
The BBC's Africa Analyst Martin Plaut says the talks in Abuja are a last effort attempt to maintain the fading credibility of the AU, while preventing the flames of war from spreading beyond the borders of Sudan.