Rebels from the Sudanese war-torn Darfur province said a truce announced by the government over the weekend has been violated.
The rebels are accusing government forces of attacking an area on the border with Chad, and say this is a violation of a unilateral cease-fire declared by the government during peace talks on Saturday in Libya.
Key rebel groups did not attend the talks, casting doubts on whether the truce would be at all effective.
More than a dozen armed rebel groups are in Darfur, fighting not only the government in Khartoum but also with one another.
Two of the rebels groups that did not attend the talks told Reuters that government forces had attacked the Jabal Moun area on the Sudan-Chad border on Saturday while the cease-fire was being declared.
The Sudanese army denied the reports.
Talks in Libya aim to stop the fighting which began in Darfur more than four years ago.
More than 200,000 people have been killed and more than 2.2 million have been displaced in the conflict.
Rebels took up arms against the central government in early 2003 to protest what they said were years of discrimination. The government is said to have unleashed armed groups called the Janjaweed to counter the rebels, but Khartoum denies this.
The failure of the two sides to reach a sustainable agreement could hamper the deployment of a joint United Nations-African Union peacekeeping force, which is scheduled to start operations next year.
Representatives of the Arab League are gathering in Khartoum on Tuesday for a two-day conference to discuss humanitarian support for Darfur. Arab League Secretary-General ‘Amru Mousa is also attending the meeting, which will focus on aid and development for Darfur.
Human rights organizations say the Arab League must use the opportunity to press the Sudanese government to end human rights abuses in the region.
Human Rights Watch said the Sudanese government must end attacks on civilians, bring perpetrators of war crimes and crimes against humanity to justice and facilitate the deployment of an international peacekeeping force in the area.