Two Sudanese aid workers were killed and one was kidnapped after their vehicle was attacked by gunmen in eastern Sudan near the Eritrean border on Sunday afternoon.
One other aid worker was seriously injured in the attack. The Sudanese army said one of the attackers was a Darfuri and four others were Rashidiya.
Rebels from Darfur, which is in its third year of open revolt, have bases in the Eritrean capital, and Sudan has accused them of having training camps near the eastern border with Eritrea.
But an SLM rebel spokesman denied involvement, saying the group had no troops in the east of Sudan or on the Eritrean side of the border.
"We as the SLM are in Darfur in the west. We are not in Kassala in the east," said SLM spokesman Adam Ali Shogar.
"Our policy is not to make any confrontations with any non-governmental organisations -- we will cooperate with them."
The Darfur rebels launched an open revolt in early 2003. Tens of thousands have been killed in the violence and more than 2 million have fled their homes in the remote western region.
Rebel leaders have said the Darfur movements had forged links with smaller eastern rebel groups.
Rebels in both the west and the east of Sudan have similar complaints against the central Arabic-speaking government. They accuse Khartoum of giving preferential treatment to Arab tribes over non-Arabs and of marginalising them politically.