"The military operations against the armed groups have enabled a very large number of children formerly used as child soldiers and in other support roles to escape," said Felix Prosper Basse, spokesman for MONUSCO, the UN peacekeeping mission in DR Congo.
"At least 430 children, among them three girls (have been) separated since January 1, 2015" from armed groups in the restive east of DR Congo, Basse said.
In just four days, from April 27 to May 1, 65 boys associated with armed groups managed to get away, Basse added.
The FDLR are Rwandan Hutu rebels whose older members are blamed for taking part in the 1994 Rwandan genocide of some 800,000 people, mainly from the Tutsi minority, before taking refuge in DR Congo.
More From This Section
The rebel group, which is opposed to Rwandan President Paul Kagame, has since January come under a major Congolese army offensive.
Basse urged armed groups to free children still fighting alongside militants, "so that they can start a normal life, go back to school, learn a trade and leave behind for good a life in the bush and crimes committed in the name of armed group commanders".
"They are children, not soldiers," he pleaded.
The UN children's agency says there may be as many as 30,000 Congolese children fighting or living with armed groups, an estimated 30 to 40 per cent of them girls.