The United Nations is calling for the military of South Sudan to release all remaining child soldiers who are serving within its ranks.

The U.N. Mission in the Republic of South Sudan (UNMISS) said Thursday that 53 child soldiers between the ages of 13 and 17 were released by the South Sudanese armed forces earlier this week in Bentiu, the capital of the country's Unity state.

The peacekeeping mission said the children were forced in April into the Sudan People's Liberation Army (SPLA), the former rebel group that later formed the bulk of the South Sudanese military when the country declared its independence in July.

The senior child protection officer of UNMISS, Hazel De Wet, said the release was a “significant positive development.” But she also urged the SPLA to release all remaining underaged soldiers, saying “children should be in schools and not military barracks.”

Child soldiers were commonly used during Sudan's 21-year north-south civil war, which left nearly 2 million people dead.

The SPLA previously committed to demobilizing all of its underaged soldiers by November 2010.