Omar al-Bashir, Sudan's president, has arrived in Qatar to conclude a peace agreement that could see an end to the war in Darfur.
Al-Bashir is expected to formally sign the peace agreement between the Sudanese government and the Justice and Equality Movement (Jem), Darfur's largest opposition group, on Tuesday, in Doha, the Qatari capital.
Leaders from both sides have promised to reach a final peace deal by March 15.
Al Jazeera's Zeina Awad, reporting from Doha, said: "There are some serious roadblocks ahead of peace talks.
"The Sudanese government is quite upbeat about the whole situation. They say that they believe they will iron out the details on the key flashpoints in time to meet their own self-declared March 15 final ceasefire with the Jem movement."
A preliminary document for the "framework agreement" was signed on Saturday in Ndjamena, Chad's capital, between representatives of the two sides, paving way for a ceasefire to facilitate forthcoming elections.
Sudan is to hold its first multiparty elections in April for the first time in 24 years.
But hours before both sides agreed to a ceasefire and signed the agreement, Sudan's army clashed with Jem fighters.
"It is unbelievable. While they were sitting down with us in Ndjamena, they were attacking us in Darfur"
Abubakr Hamid Nur,
The clashes underline the challenges facing efforts to end the conflict.
"The government troops attacked our forces just after midday," Abubakr Hamid Nur, a Jem field commander, told the Reuters news agency on Saturday.
"It is unbelievable. While they were sitting down with us in Ndjamena, they were attacking us in Darfur."
A UN source, speaking on condition of anonymity, said there were clashes involving Jem and Sudan's army in the area on Saturday, but could not confirm who attacked or won.
Fighting has intensified in the run up to past ceasefires and negotiations on Darfur as warring parties try to maximise territorial gains ahead of settlements.
Ahmed Hussein Adam, a Jem spokesman, said the rebel force regretted the incident but said it could have been caused by a breakdown in communication between Sudan's negotiators and their forces in the field.
"This is something we can put behind us. Everyone here wants to enter into the new spirit." He said the fighting ended before the deal was signed.Peace talks
A spokesman for Sudan's army dismissed Jem's report, telling Reuters: "This story is absolutely wrong. Sudanese army didn't attack this area alone or with other forces."
Last year, Sudan's government and the Jem rebels signed an agreement in Qatar, a step toward ending a six-year conflict that has killed hundreds of thousands.
Qatar has been mediating talks between the two sides in the Darfur conflict, which erupted in 2003 after rebels began an uprising against the Khartoum government.
Al-Bashir is under pressure to end the fighting, particularly because he was charged with seven counts of war crimes and crimes against humanity in Darfur by the International Criminal Court (ICC) last year for the government's campaign of violence in Darfur.
Qatar is not a member of the ICC and would have no legal obligation to arrest al-Bashir on its territory.
International experts say at least 200,000 people have been killed in Darfur and more than 2.7 million driven from their homes in almost six years of fighting.
Khartoum disputes the figures and says 10,000 people have died.
The conflict began when rebels took up arms against the government saying their region was being marginalised.
A Sudanese court condemned 105 members of Jem to death after the group launched an assault in May 2008 that reached Khartoum's twin city of Omdurman, just across the Nile from the presidential palace.
Darfur's other main rebel group, the Sudan Liberation Army (SLA), is refusing to talk to the government, demanding an end to all violence before negotiations begin.