South Sudan government and rebel forces are both recruiting children as young as 13 to fight in the country's civil war, a rights group said Monday in a report rejected by the government.

The government is "actively recruiting" teenagers, often by force, while rebels are also using child soldiers in the 14-month long conflict, said Human Rights Watch.

"Despite renewed promises by both government and opposition forces that they will stop using child soldiers, both sides continue to recruit and use children in combat," said HRW's Africa director Daniel Bekele.

Minister for Information Michael Makuei dismissed the report, saying there were plenty of men still able to fight.

"How can we recruit child soldiers at a time when we have sufficient manpower?" he told AFP. "We have no child soldiers."

But HRW say that government forces are taking children, in some cases, such as in the northern war-damaged town of Malakal, "from right outside the United Nations compound."

Malakal, in oil-rich Upper Nile State, has changed hands six times since the war started in December 2013, and 21,000 civilians have sought shelter and safety behind the UN's barbed-wire fences.

The ruined city is currently under government control and HRW said some children were forcibly recruited from outside the gates of the UN base.

The UN children's agency says that in the last year 12,000 children, most of them boys, have been recruited and used as soldiers by the army, rebels and allied militias.

Since the start of the year UNICEF has negotiated the release of 3,000 child soldiers from a rebel group commanded by David Yau Yau in the Pibor region of Jonglei, but many more still are fighting and recruitment is continuing.

In response to international pressure South Sudan's government passed a 2008 law banning the use of child soldiers and setting a minimum age of 18 for recruitment or conscription.

President Salva Kiir and rebel leader Riek Machar headed the government that signed the 2008 law, but it was quickly flouted when fighting began and soldiers were needed.

Fighting broke out in December 2013 when Kiir accused Machar of attempting a coup, setting off a cycle of retaliatory killings across the country. War continues despite numerous ceasefire deals.