The recruitment of child soldiers in South Sudan has spiked in the last week as the government prepares for a renewed civil war, an internal United Nations document said according to the Associated Press.

A whole village of boys was pressed into joining the army by a senior politician acting on President Salva Kiir's instructions, the document suggests. Although it is not clear how many were involved, some were as young as 12 years old.

Fierce fighting broke out in the capital Juba last month, killing hundreds of people, and raising fears of more violence in the world's youngest country.

The United Nations' children's agency UNICEF said more than 16,000 children have been recruited into armed groups since December 2013 when the civil war first erupted.

"The dream we all shared for the children of this young country has become a nightmare," said UNICEF's deputy executive director Justin Forsyth.

Army spokesperson Lul Ruai Koang denied that children were forced into joining, adding that he was not aware of any recent recruits.

The tensions exist between President Sava Kiir and his former vice president Riek Machar.

The ethnically charged war has forced more than one in five of South Sudan's 11 million people to flee their homes and around half of the country's children do not attend school.

Despite a peace deal in August 2015, renewed fighting has raised fears of another civil war.

Machar, who resumed his role as vice president under the 2015 peace deal, withdrew from Juba with his soldiers last month and left South Sudan on Wednesday to escape government forces, his party said.

UNICEF has said an additional 650 children have joined military forces since the beginning of 2016 in preparation for further violence.

Last month's fighting in Juba bought "horrific ordeals" as government soldiers executed civilians and gang-raped women and girls, said UNICEF. The South Sudan government denied the allegations.

"The systematic use of rape, sexual exploitation and abduction as a weapon of war in South Sudan must cease, together with the impunity for all perpetrators," Forsyth said.