JUBA - Two UN agencies said Monday that over 1 million children have fled South Sudan due to escalating armed conflict.

Latest figures by UN Children's Fund (UNICEF) and the UN Refugee Agency (UNHCR) show that children make up 62 percent of the more than 1.8 million refugees from South Sudan and that most have arrived in Uganda, Kenya, Ethiopia and Sudan.

"Refugee families fleeing to neighboring countries in search of shelter and safety are facing a double catastrophe this rainy season, with children most at risk from the health and protection risks associated with inadequate shelter," the UN agencies said in a joint statement.

The agencies said children remain at risk of recruitment by armed forces and groups and, with traditional social structures damaged, are also increasingly vulnerable to violence, sexual abuse and exploitation.

South Sudan has faced ongoing challenges since a political face-off between President Salva Kiir and rebel leader Riek Machar erupted into full-blown conflict in December 2013. The crisis has produced one of the world's worst displacement situations with immense suffering for civilians.

Despite the August 2015 peace agreement that formally ended the war, conflict and instability have also spread to previously unaffected areas in the Greater Equatoria and Greater Bahr-El-Ghazal regions of South Sudan.

The UN agencies said over 75,000 refugee children in Uganda, Kenya, Ethiopia, Sudan and DR Congo have crossed South Sudan's borders either unaccompanied or separated from their families.

UNICEF's Regional Director for Eastern and Southern Africa Leila Pakkala said the shocking fact that nearly one in five children in South Sudan has been forced to flee their home illustrates how devastating this conflict has been for the country's most vulnerable.

The UN agencies said over 1,000 children have been killed or injured since the conflict first erupted in 2013 in South Sudan, while an estimated 1.14 million children have been internally displaced.

Nearly three quarters of the country's children are out of school, the highest proportion of out-of-school children in the world, the agencies said.

Valentin Tapsoba, UNHCR's Africa Bureau Director, said the humanitarian community need most urgent, committed and sustainable support to be able to save their lives.

"No refugee crisis today worries me more than South Sudan. That refugee children are becoming the defining face of this emergency is incredibly troubling," Tapsoba said.

The UN agencies said the trauma, physical upheaval, fear and stress experienced by so many children account for just part of toll the crisis is exacting.

"Much greater support is needed to ensure that every refugee family has somewhere safe to live, as well as urgent humanitarian assistance including food, water, protection, education and medical care," the agencies said.