Sudan's president is due to sign a ceasefire with Darfur rebel group the Justice and Equality Movement (Jem).
Omar al-Bashir will sign a framework agreement which includes an outline deal on power sharing "at all levels".
It is being seen as an important step towards peace, though the other main rebel group has refused to enter talks.
But Jem was reported to have warned that it was unlikely to sign a final peace accord by mid-March, ahead of national elections in April.
"After the agreement is signed, the rest will come through more negotiations," Adrees Mahmoud, a Europe-based representative of the group, was quoted as telling Reuters news agency.
He added that it was too early to say if the target of a final binding agreement by March - one of the clauses of the ceasefire deal - would be met.
Mr Bashir is due to sign the accord in Qatar, where it was brokered in negotiations by neighbouring Chad. Sudan says Chad has backed and armed Jem.
According to the text of the accord, obtained by the BBC, the rebel group will constitute a political party after the signing of a final agreement.
As well as an immediate ceasefire, the agreement includes an outline deal on the sharing of power "at all levels", which means the rebels will be offered seats in the Khartoum government.
The text also specifies that changes in the administration of the Darfur region will form part of the final accord and that death sentences imposed on 100 Jem fighters will be cancelled.
The International Criminal Court (ICC) last year issued a warrant for Mr Bashir's arrest for war crimes in Darfur.
But Qatar has not signed the ICC charter, which obliges member states to arrest indictees on their territory.
'Redouble peace efforts'
The BBC's James Copnall, in the Sudanese capital, Khartoum, says the deal is a major breakthrough.
The power-sharing agreement, our correspondent adds, has shaken the political establishment.
Two years ago, the Darfur rebel group took its fight to the heart of the country, attacking the city of Omdurman, where parliament sits.
The government said that more than 200 people were killed in the attack and sentenced more than 100 Jem fighters to death by hanging for their involvement.
The seven-year war between forces loyal to the government and rebels in Darfur has lost intensity in recent years.
But the UN estimates 300,000 died in the worst years of the conflict. Some 2.5 million people are still displaced.
The UK's Africa minister, Glenys Kinnock, hailed the ceasefire and power-sharing agreement, while urging all sides involved to "redouble their efforts for peace".
Meanwhile, in Southern Sudan, several days of fighting in the Lakes State region has killed at least 28 people, including seven soldiers.
The fighting started when Gok Dinka gunmen attacked a military base in an attempt to seize more weapons, following fighting with the rival Rek Dinka clan.
It was only the latest in a string of violent clashes between rival ethnic groups and the army, reports the BBC's Peter Martell from the southern Sudanese capital Juba, and the scale and the frequency of those is worrying many.
Although the civil war with the north ended in 2005, some 2,500 people died in conflicts between rival communities in Southern Sudan last year - far more than in Darfur, the UN says. - BBC