By Denis Dumo

JUBA May 9 (Reuters) - Tens of thousands of people have fled fierce fighting in South Sudan's northern Unity State and humanitarian organisations have withdrawn staff from the area, the International Committee of the Red Cross (ICRC) and other groups said on Saturday.

A political crisis in South Sudan in late 2013 sparked fighting between forces loyal to President Salva Kiir and rebels allied with his former deputy Riek Machar. The conflict in the world's youngest country reopened ethnic fault lines that pit Kiir's Dinka people against Machar's ethnic Nuer forces.

A government military spokesman, Philip Aguer, confirmed the fighting in Unity State. A spokesman for the rebels could not immediately be reached for comment.

Doctors without Borders said it had shut down a hospital in the town of Leer amid reports of an imminent attack.

The group said it had closed the same facility last year when staff members fled on foot, carrying critically ill patients on their backs. They hid on the banks of swamps and survived by drinking swamp water, it said in a statement.

"Today, we withdraw again with a heavy heart, because we know how civilians will suffer when they are cut off from critical, lifesaving medical care," said Paul Critchley, head of the mission.

The International Committee of the Red Cross (ICRC) said separately it had also been forced to withdraw key staff from Leer and that it was concerned for the wellbeing of tens of thousands of people who have reportedly fled the area.

"These communities face a fight for survival, hiding in the bush in unimaginably harsh conditions," said Franz Rauchenstein, who heads the ICRC's delegation in South Sudan.

The fighting could also prevent farming communities from planting much-needed crops during the imminent rainy season, he said. (Writing by Edith Honan; Editing by Raissa Kasolowsky)