President Barack Obama announced Monday that the United States would offer an extra $50 million to help tackle a growing refugee crisis spawned by fighting in South Sudan.

The money will be sent to international and non-governmental organizations and be used to pay expenses of the State Department's Bureau of Population, Refugees, and Migration in the region, Obama said in a memorandum.

"Months of conflict between the government of South Sudan and rebel forces have exacted a terrible toll on the people of South Sudan," said National Security Council spokeswoman Caitlin Hayden.

"More than 1.3 million people have been displaced, and the country is now at risk of famine unless humanitarian agencies are able to scale up relief efforts and access those in need."

Hayden said the $50 million in extra emergency aid would be part of a $300 million grant the US delegation will formally unveil at a pledging conference in Oslo on Tuesday and would bring total US humanitarian assistance since the start of the conflict in South Sudan last year to around $433.6 million.

The United Nations says $1.26 billion is needed in conflict-torn South Sudan to avoid a major humanitarian crisis threatening millions of people.

South Sudan became independent in 2011 after a bloody war between Sudan and the Sudan People's Liberation Movement (SPLM) rebel group currently in power.

The fledgling country was plunged back into an ethnic conflict between its two main groups the Dinka and Nuer following a dispute between President Salva Kiir and his former vice-president Riek Machar in July 2013.

Despite the signing of a peace agreement on May 9, hostilities have continued, resulting in massacres and atrocities against thousands of civilians from both sides.