Geneva (AFP) - Up to 16,000 children have been forced to join the fighting in South Sudan since the start of a civil conflict in 2013, the United Nations children's agency (UNICEF) said Friday.

Despite an peace agreement in August, fighting continues to rage in the two-year civil conflict in the world's youngest nation, with children suffering an increasingly heavy toll.

"There are now 16,000 children associated with armed groups and the military," UNICEF spokesman Christophe Boulierac told reporters, adding that minors continued to be killed, kidnapped and subjected to sexual violence.

Boulierac said that some children who have joined armed groups have been forced into direct armed combat, while others are put to work as messengers or porters in extremely dangerous circumstances.

Since the fighting broke out in December of 2013, an estimated 1,500 children have been killed, 900,000 have been internally displaced and one million children are suffering from malnutrition, according to UNICEF.

"Despite the signing of the peace deal in August, there has been little sign of improvement," the UNICEF spokesman said.

The civil war began when President Salva Kiir accused his former deputy Riek Machar of planning a coup, setting off a cycle of retaliatory killings that have split the poverty-stricken, landlocked country along ethnic lines.

On Thursday, ceasefire monitors from the regional IGAD bloc issued a fresh call to warring forces to stop fighting amid growing fears that the peace deal was close to collapse.

The conflict has triggered a humanitarian crisis with 2.3 million people forced from their homes and 4.6 million in need of emergency food assistance. Tens of thousands have died and the economy is in ruins.