Lord's Resistance Army rebels are leaving their bases in the Democratic Republic of Congo as demanded by a peace deal with Uganda's government.
LRA leader Joseph Kony is wanted by the ICC for war crimes
The truce, signed on Saturday, has a ceasefire coming into effect on Tuesday and gives rebels three weeks to move into assembly points in southern Sudan.
The Ugandan government has pledged that it will not try to attack the rebels.
Thousands have died during the 20-year conflict in northern Uganda, and more than one million have fled their homes.
Under the terms of the truce signed by both sides, the rebels will leave Uganda and their bases in Sudan and DR Congo to gather at two assembly points, where they will be protected by the government of southern Sudan.
More than 1m Ugandans have fled their homes
Talks on a comprehensive peace agreement will then get under way.
Ugandan President Yoweri Museveni has set a 12 September deadline for a final peace deal.
Villagers in DR Congo said they had seen groups of Ugandan rebels marching from a national park towards the Ugandan border.
Meanwhile, rebel leader Joseph Kony and his senior commanders are gathering on Congolese territory near the border of the Central African Republic.
Diplomats say the plan is to fly them to the assembly points in Sudan.
The International Criminal Court (ICC) wants the LRA's top officials - among them Joseph Kony - to face charges including murder, rape and forcibly enlisting children.
The LRA has abducted thousands of children and forced them to fight since the conflict began.
Against the wishes of the ICC, Uganda offered amnesty to LRA leaders in exchange for the peace talks.