Two staff members of the foreign aid group Tearfund were killed when their convoy was attacked in eastern Congo, the organization said, the latest in a series of attacks targeting aid groups and residents in the region.

The aid workers were killed on Sunday after their convoy had arrived in Butembo city in North Kivu province, where armed rebels have been fighting Congolese security forces, the organization said in a statement late Monday.

It wasn't immediately clear which group was behind the attack. More than 120 armed groups in eastern Congo have been carrying out attacks, often involving bombs targeting residents, as they seek a share of the region's gold and other resources.

More than 170 such security incidents have targeted humanitarian workers, causing at least four deaths and the abduction of more than a dozen people, the United Nations Office for the Coordination of Humanitarian Affairs in Congo said Tuesday in a statement that called for the protection of aid workers.

"At a time of immense humanitarian need, it is unacceptable that those working to help affected people are being attacked and killed," said Bruno Lemarquis, the humanitarian coordinator for Congo.

Violence in the province has worsened in recent months as security forces battle the rebels. Two people were killed last week in the region when mortar shelling targeted a base operated by South Africa's military, which is part of a regional peacekeeping mission. The rebels have also taken over more villages in recent weeks.

The M23 rebel group, with alleged ties to neighbouring Rwanda, has been the most active in the region, seizing strategic towns, with about half of North Kivu province under their control, according to Richard Moncrieff, the Crisis Group's Great Lakes region director.