The National Movement for Reform and Development (NMRD) signed a ceasefire with Khartoum in December last year and agreed to a security deal in April.
The group split last year from one of the two main Darfur rebel groups, which are due to start talks with the Sudanese government in Abuja on Thursday.
"The ceasefire is over," NMRD Political Secretary Khalil Abd Allah said on Wednesday.
"We will resume our operations on the ground because we are not a part of the negotiations," he said, adding that the government had failed to implement the ceasefire signed with the group.
"We are representative of the people of Darfur. Without us, there is no serious chance for peace," he said.
Government forces and allied militia had attacked NMRD's positions on numerous occasions since the signing of December's ceasefire agreement, Abd Allah added.
The group is said to be smaller than the Justice and Equality Movement (JEM) and the Sudan Liberation Army (SLA), the groups which launched the Darfur rebellion in early 2003, but Khalil said it had the same number of fighters as the SLA.
He said the European Union and the United Nations needed to get involved in the peace process to solve the crisis in Darfur, where tens of thousands have died and around 2 million have been forced from their homes.
Peace talks are to resume in
Nigeria on Thursday
"The AU doesn't know what is happening in the field ... The time for the AU is over," Khalil added.
The talks, the sixth round of Darfur peace negotiations, appeared unlikely to start on time as none of the key negotiators had arrived in Abuja by Wednesday afternoon.
SLA Chairman Abdel Wahed Mohamed al-Nur previously said his group would go to the talks despite government attacks aimed at derailing them.
Al-Nur planned to leave for Abuja from Asmara with his delegation on Thursday after confusion over flight tickets, an SLA official said.