Khartoum - Security in Darfur is worsening and militias need to be disarmed, the US charge d'affaires to Sudan Joseph Stafford says, while condemning the recent attack which killed a peacekeeper.
Stafford also expressed grave concern on Sunday "about the reports of civilian casualties and deliberate targeting of civilians by militias in Muhagiriya and Labado", the region where the Nigerian peacekeeper died on Friday.
He told reporters it was not yet clear who carried out the "deeply troubling" attack against the base of the African Union-UN Mission in Darfur (Unamid) near Muhagiriya town in southern Darfur.
But he said there was an "urgent" need for an investigation bringing those responsible to justice.
Two other peacekeepers were wounded in the assault, two days after the Sudanese government announced it had regained control of the area from rebels.
"We're worried about the deteriorating security situation in Darfur and the conflict between the government forces and the militia", Stafford said.
He said "escalation in the acts of violence underscores the need for militia disarmament" as well as other measures including a ceasefire between the government and rebels who have not signed a 2011 peace deal with Khartoum.
That deal is a good foundation but "implementation is proceeding slowly", said Stafford, whose country says it is Darfur's largest single aid provider.
Under the peace deal between the government and rebel splinter factions, Khartoum is responsible for disarming all militia.
Last October a monitoring committee set up under the peace pact said authorities had filed a comprehensive plan to disband the groups.
More than 40 peacekeepers have been killed in hostile action during Unamid's five-year history and the UN has repeatedly called for perpetrators to be brought to justice.