Two aid workers have been killed in Sudan's war-torn Darfur region, says Save the Children UK.
More than two million have been displaced by the violence in Darfur
The vehicle the two Sudanese nationals were travelling in came under fire in South Darfur. The group has suspended aid operations there.
The number of ceasefire violations has been on the rise in recent months, an African Union spokesman warned.
On Monday, rebel leaders accused the government of breaching the truce and said they would boycott peace talks.
The rebels announced that they would stay away from the talks in the Nigerian capital, Abuja, until the government promised to halt attacks.
Sudan Liberation Movement spokesman Bahar Ibrahim said: "We are suspending the talks until the situation has changed and there is a clear commitment that the Sudanese government will stop the offensive."
The other rebel group, the Justice and Equality Movement, said it adopted the same position.
However, both groups said they were not leaving Abuja immediately and were open to informal consultations, Reuters news agency reported.
Some 70,000 people have been killed and some two million have fled their homes in almost two years of violence between black Africans and pro-government Arab militias and the army.
Save the Children UK says Abhakar el Tayeb, a medical assistant, and mechanic Yacoub Abdelnabi Ahmed, were travelling in a clearly marked humanitarian convoy.
In October, two Save the Children workers were killed by a landmine in North Darfur.
Save the Children says it is not clear who was responsible for the killing but African Union military observers are investigating.
A spokeswoman said that they had health clinics and feeding centres in the area's camps which served tens of thousands of people who had fled their homes.
Thirteen violations of a ceasefire agreement were confirmed in September and 54 documented between October and mid-December, said Assane Ba, a spokesman for the African Union mediating the talks in Abuja.
"That means the violations are growing," he said. "This is poisoning the atmosphere and we can't have meaningful negotiations in this situation."
The United States and human rights groups say a genocide is being committed against Darfur's non-Arab groups and accuses the government of arming the militias.
Sudan's government says the rebels are responsible for the situation after taking up arms in February 2003.