KAMPALA, Uganda - About 3,000 children will be gradually released from an armed group in South Sudan, the United Nations children's agency said after a group of 280 children were freed on Tuesday.

More phased releases will happen in the coming weeks, said UNICEF, which helped negotiate the freedom of the children, who range in age from 11 to 17.

"These children have been forced to do and see things no child should ever experience,"UNICEF South Sudan Representative Jonathan Veitch said in a statement. "The release of thousands of children requires a massive response to provide the support and protection these children need to begin rebuilding their lives."

They were recruited by the South Sudan Democratic Army Cobra Faction, an armed group whose leader David Yau Yau signed a peace agreement with the South Sudan government last year.

Last year 12,000 children were used as soldiers by armed forces and groups across South Sudan, said UNICEF.

Although Yau Yau's rebellion has since been contained, South Sudan remains prone to fresh outbreaks of violence because of a rebellion led by the former deputy president.

The international community will seriously consider sanctions on South Sudan's warring factions if they don't reach a power sharing agreement this week, said Norway's foreign minister Borge Brende.

South Sudan's warring factions are expected to reach an agreement in peace talks planned alongside the African Union summit in Ethiopia on Thursday, said Brende, who's attending a preliminary A.U. executive council meeting in Addis Ababa.

Brende's statement came days after the U.S., U.K. and Norway said they are gravely concerned with the lack of progress in the South Sudan peace negotiations.

Fighting broke out December 2013 between South Sudan President Salva Kiir's troops and those loyal to Riek Machar, Kiir's former vice president. Since then, more than 1.5 million South Sudanese have fled their homes.

The two sides have signed peace agreements brokered in Ethiopia but fighting has continued.