"Two staff members have been shot down, a third was injured and a fourth was abducted and is still missing," Asaf Bukhari, SRC spokesperson, said on Wednesday. "The attackers were driving a car when they suddenly opened fire on the SRC vehicle."

The SRC driver and a nurse died on the spot from their wounds, the International Committee of the Red Cross (ICRC) said in a separate statement, released on Tuesday.

An SRC medical assistant was severely injured in the attack and was recovering in a hospital, the ICRC said. A fourth man, also a passenger in the car, remained missing.

The Sudanese army, Bukhari said, had found the vehicle and suspected that most of the attackers were from the local Rashida tribe. One attacker reportedly came from the western Sudanese region of Darfur.

"The SRC met with the Sudanese minister of humanitarian affairs today, and raised the issue with him," Bukhari said. "We are deeply concerned about the safety of our volunteers who are working in Kassala and in southern Sudan."

Bukhari said the attackers had not yet been apprehended, and the SRC had not been in contact with the Rashida tribe.

A small rebel group from the non-Arab Beja tribe has been active in the poor region near the Eritrean border, but fighting has been sporadic and on a small scale.

On 16 January, the Sudanese government reached a preliminary political settlement with the National Democratic Alliance (NDA), an alliance of opposition groups, including the Beja Congress, in Cairo.

The deal supported the peace accord signed with the southern Sudan People's Liberation Movement/Army (SPLM/A) on 9 January, and backed the drafting of a new constitution and calls for the formation of a neutral, professional army.

Although the agreement was hailed as an important step towards nationwide peace and the disarmament of the NDA's armed militias, the Beja Congress, an exiled group representing numerous eastern Sudanese tribes, boycotted the Cairo talks.

The NDA had for the past 16 years fought alongside the SPLM/A in the southern civil war, launching attacks in the north and east against the Khartoum government.

The Beja Congress accuses the government of marginalising the remote regions of the country and demands greater autonomy. They see the agreement between the government and the SPLM/A as a model for their own region.

Originally a nomadic people, many Beja live in shantytowns on the outskirts of Port Sudan. They moved to the port to work as labourers after famine killed their cattle and mechanised farming took over their lands in the 1980s.