International Criminal Court officials are travelling to Rwanda to collect Congolese warlord Bosco Ntaganda, who is holed up in the US embassy there, a top American diplomat said Wednesday.

Ntaganda surprised US embassy staff in Kigali on Monday when he walked in off the street and asked for help in reaching the ICC in The Hague, where he is wanted on charges of war crimes and crimes against humanity.

"Officials from the ICC are, as we speak, en route to Kigali," the top US diplomat for Africa, Johnnie Carson, told reporters on Wednesday.

"The timeline is uncertain but the need for rapid and quick action is clear," Carson added, speaking on a conference call from Washington.

"The next 48 hours or so will be critical in all this," he added.

Carson, Assistant Secretary of State for African Affairs, said that Ntaganda "voluntarily walked in" to the embassy, but had no clear answer as to why he chose a United States diplomatic mission for his surrender.

"I suspect that he may have come because he knows that we are a symbol of fairness and justice and integrity in this kind of process... but I don't know and can't read his mind," he added.

Carson also appealed to Rwanda to allow Ntaganda free passage to the airport in Kigali on his way to trial "without interference".

Carson said there had been "very open and good contact" with Rwandan officials, who have given assurances they will allow Ntaganda to go to The Hague. But he also said the "realities in practical terms" of how Ntaganda would travel to the airport were still to be ironed out.

Ntaganda was allegedly involved in the brutal murder of at least 800 people in villages in the eastern Democratic Republic of Congo, using child soldiers in his rebel army and keeping women as sex slaves between September 2002 and September 2003.

Carson said bringing Ntaganda to The Hague would send a "clear signal" to other rebel leaders and be a step towards improving the situation in DR Congo's volatile east.

"It will take off the battlefield one of the most notorious rebel leaders, a man dubbed by the media 'The Terminator'," Carson said. AFP