UK-based charity Save the Children is pulling out of the troubled Darfur region of western Sudan after attacks that have killed four of its staff.
Rafe Bullick (right) is among the charity's employees killed recently
The charity said it was "devastated" it could no longer serve some 250,000 children in the area but the risks facing its workers were "unacceptable".
Thousands continue to pour into refugee camps amid fresh fighting in Darfur.
The latest attack on the agency saw two of its 350-strong Darfur team shot dead on a highway on 12 December.
According to the charity, the workers were travelling in a convoy of clearly-marked humanitarian vehicles when they were shot.
Two Save the Children staff were killed in October when their vehicle hit a landmine.
The charity is one of the larger agencies operating in Darfur.
Mike Aaronson, the charity's UK director, told BBC News that world leaders must effect a ceasefire deal before setting out to solve Darfur's political problems.
Decrying the region's "endless ceasefire violations" and its "atmosphere of increasing lawlessness", he called for a stop to "foot-dragging" on the UN Security Council and for more African Union troops to be committed on the ground.
Helicopter fired on
Hundreds of thousands of displaced people remain beyond the reach of humanitarian organisations, says our Khartoum correspondent, Jonah Fisher.
The UN has termed Darfur one of the world's worst humanitarian crises, with some two million refugees relying on aid handouts for their survival.
Tens of thousands of people have been killed in almost two years of clashes between rebels, the army and militia groups known as the Janjaweed.
Khartoum has denied allegations it is backing the Janjaweed.
A small African Union force monitoring a shaky ceasefire between the warring groups was forced to stop flights over southern Darfur after one of their helicopters was shot at. It is now undergoing repairs. Their other helicopter is still operational in the west and north of the region.
Some 200 Gambian troops are due to join the force, but there are still less than 1,000 - a third of the agreed size - to cover an area the size of France.
The government has promised to halt a two-week offensive in the south.
Darfur rebels and Sudanese government representatives have adjourned African Union sponsored peace talks until January, with both sides reaffirming their commitment to cease hostilities.
Nigerian President and AU chairman Olusegun Obasanjo urged them to observe an April ceasefire and move back to positions held when the truce was signed.
He said the government had reaffirmed its commitment to a ceasefire and to withdraw its troops to their previous positions. The two main rebel groups promised to stop all attacks on humanitarian and commercial activities in Darfur, and to restrain their troops from fighting with government forces.
Meanwhile, the Sudanese parliament has voted to extend the country's five-year state of emergency by a further year.
Reports from Khartoum say the government justified its decision by citing the security situation in Darfur and what it said was the threat posed by opposition groups to the country's oil infrastructure in the north and east.
The Sudanese government is still negotiating with rebels from the south, to try and establish a final ceasefire agreement by the end of the year.