South Sudan must uphold promises to abolish its use of child soldiers, with some 2,000 minors still serving in its rebel turned regular army, the UN's top expert on children in conflict said Friday.

The South's military, the Sudan People's Liberation Army (SPLA), repeated pledges again this week -- but the first time as a national army following the South's independence in July -- to release children from its ranks.

In 2003 the SPLA was put on a UN blacklist of forces who use child soldiers, when the then guerrilla army was battling the Khartoum government in Sudan's bloody 1983-2005 civil war.

"It is very important that we delist them as soon as possible, and now they are a national army, it becomes extremely important," UN Special Representative for Children and Armed Conflict Radhika Coomaraswamy told reporters.

"If you're a violator that's been persistent, there's the possibility of sanctions," she added, noting those could include asset freezes, arms or travel embargoes.

However, she also said the fledgling nation had been making progress, including demobilising some 3,000 children since the civil war ended seven years ago, and supporting SPLA promises to release the rest.

"We estimate that there will be about 2,000 children that will be released," Coomaraswamy said, adding that many of those remaining came from rebel militia forces recently integrated into the army.

Although there have been no exact figures on South Sudan's army since independence, it is thought that it numbers somewhere between 100-150,000.

The United Nations classifies a child soldier as anyone under the age of 18.

South Sudan, oil-rich but grossly underdeveloped, is riven by brutal ethnic violence, while the government has sought to quell multiple rebellions by offering to integrate fighters into the army's ranks.

However, Coomaraswamy said the reintegration of former child soldiers was a huge challenge in a nation where over 80 percent are illiterate, and where the bloated army is the only source of paid employment for many.

Following South Sudan's slashing of budgets following a drastic shut down of oil production in January -- providing 98 percent of revenues -- Coomaraswamy said education must be supported to keep children out of armed groups.

"Even if there is an austerity budget, education should be a priority," she said.