Pressure is growing on the warring parties from Sudan's Darfur region as another extension was granted for them to secure a deal at talks in Nigeria.
Mediators hope the extra time will lead to more concessions
The US and UK have joined the African Union's mediation efforts to reach agreement before midnight on Thursday.
AU adviser Alex de Waal warned that a failure to do so could result in a regional war as the conflict worsens.
Sudan's government has agreed to sign the peace plan, but rebels want security and power-sharing concessions.
A deadline at the talks in the Nigerian capital, Abuja, ended at midnight on Tuesday but has been extended for another 48 hours, allowing the US Deputy Secretary of State Robert Zoellick and UK's International Development Secretary Hilary Benn to hold more meetings with the government and rebels.
More than 2m people have been displaced by the violence and some 200,000 people have died since the conflict began three years ago when black African rebels took up arms against Khartoum.
The Arab-dominated government responded with a scorched earth policy and pro-government Janjaweed militias have been accused of committing genocide in their crackdown on the rebels - a charge Khartoum denies.
Mr de Waal said the situation on the ground in Darfur was deteriorating and becoming more complicated.
"If this deal is not signed this week there is a very, very serious danger of a major regional war unfolding which would make the situation in Darfur probably insoluble," he told the BBC's World Today programme.
Mr Zoellick, who - along with Mr Benn - has held extensive talks with all sides, said: "I believe there has to be an end to this process."
Four parties are at the talks in Abuja: the Sudanese government, the pro-government Arab Janjaweed militias, and two rebel groups, the Sudanese Liberation Movement (SLM) and the Justice and Equality Movement (Jem).
Jem spokesman Ahmed Hussain Adam welcomed the mediation input of the US and US.
"I'm still very hopeful. I think their efforts will bring something that will get us out of this dilemma," he said.
AU Chairman and Republic of Congo's president, Denis Sassou-Nguessou, along with other African heads of state in Abuja attending a health conference are also expected to add their weight to the negotiations on Wednesday.
The BBC's Alfred Taban in Sudan's capital, Khartoum, says the deadline extensions are to be expected.
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
Last year's successful peace deal with southern rebels, which ended more than 20 years of war, was subject to many extensions too, he said.
Lazaro Sumbeiywo, the chief negotiator of that agreement, urged patience in Abuja.
"Never ever give the Sudanese a deadline. This is what I learnt during my negotiations," he told the BBC's Network Africa programme.
One of the key stumbling blocks to a deal concerns the Janjaweed, who the rebels want to be completely disarmed.
Mediators hope that the proposed deal can be amended to increase the number of rebels integrated into the army, while dropping the provision that the Janjaweed disarm before the rebels.
The rebels are also said to be unhappy about arrangements concerning power-sharing and wealth distribution in the vast desert region.
In public, some of the rebels are holding out for a regional Darfur government.
But the BBC's World Affairs correspondent Mark Doyle says Khartoum sees this as the thin end of a dangerous trend and is resisting.
In the southern peace deal, SPLM rebels gained control of some ministries in Khartoum and a share of Sudan's oil wealth.