Uganda's government and the United Nations are appealing for $8 billion to deal with the crisis of refugees from South Sudan.

In a statement on World Refugee Day, the U.N. children's fund said the money is needed for emergency aid as well as "resilience interventions" targeting both refugees and host communities over the next four years.

Most of the refugees are women and children, the reason UNICEF is calling the refugee influx "a children's crisis."

"More than one million children have been forced from their homes in South Sudan, often amid horrific violence," said Leila Pakkala, a UNICEF official in the region. "Day after day, week after week, they are being received by countries such as Uganda, Ethiopia and Kenya. Despite great efforts on many fronts, the systems in these countries are tremendously stretched."

UNICEF in Uganda requires nearly $50 million in 2017 as well as $30 million each year from 2018 to 2020 to provide critical services, including education and child protection, to both refugees and host communities, the UNICEF statement said.

This week Uganda will host a U.N.-backed summit aimed at raising awareness of the refugee crisis.

More than 1.8 million people have crossed into neighboring countries since the start of South Sudan's conflict in December 2013. More than 900,000 of them are sheltering in Uganda, on plots of land often donated by Ugandans.

Amnesty International charged on Monday that rich countries have failed in their obligation to help Uganda support thousands of refugees fleeing violence in South Sudan.

The rights group urged donors, including the United States and the European Union, to ramp up refugee support, saying it had spoken to refugees and aid agencies who spoke of a desperate lack of food, water, shelter and other basic services.

Uganda's refugee policy has been called one of the most progressive in the world because refugees are allowed freedom of movement and can operate their own businesses.

Yet South Sudan's refugee crisis is testing the limits of Uganda's generosity, with the continuing influx exerting a lot of pressure on the available resources.