One of the biggest rebel groups in Sudan's Darfur region is boycotting peace talks with the government, accusing it of launching new raids.
The Justice and Equality Movement said the army had carried out air strikes and attacked villages in contravention of a ceasefire signed in February.
An army spokesman said its forces had come under attack.
Darfur was relatively peaceful during the April elections, which saw President Omar al-Bashir re-elected.
But voting did not take place in much of the area because of the insecurity.
The International Criminal Court has issued an arrest warrant for Mr Bashir over alleged war crimes committed in Darfur - charges he strongly denies.
Jem official Ahmed Tugod Hassan told the BBC that the fighting took place in the Jebel Moon area of West Darfur.
Rebel spokesman Ahmed Hussein Adam told Reuters news agency that the group was freezing its participation in the talks in Doha.
"The [Khartoum] regime either has to choose the path of war or the path of peace. If the offensive continues against our people we consider taking further measures as well," he said, without giving details.
But the BBC's James Copnall in Khartoum says this could be empty posturing, as Jem's military options are limited.
Weeks after Jem signed a ceasefire, a second rebel coalition - the Liberation and Justice Movement (LJM) - did likewise, paving the way for peaceful elections.
A deal with this group appears more likely than with Jem, our correspondent says.
The only major group still holding out against the government is a faction of the Sudan Liberation Army led by Abdul Wahid, who has refused to take part in peace talks.
An advisor to President Bashir has said the government will probably unveil a new policy towards Darfur following his re-election.
Ghazi Saleheddine said the focus would move from talks with rebel movements to development inside Darfur.
But our correspondent says many in Darfur, including rebels and the millions living in camps for displaced people, will be deeply sceptical.
Since the conflict in Darfur began in 2003, some 2.7 million people have fled their homes and the UN says about 300,000 more have died.
The rebel movements have been fighting government soldiers and Arab militias who many people say are backed by the government.
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