By Katy Migiro

NAIROBI, Feb 9 (Thomson Reuters Foundation) - Donors pledged more than $500 million in aid to South Sudan on Monday, the United Nations aid chief said, describing her visit to the country as painful because of the suffering of the people she met.

One in five South Sudanese -- 2.5 million people in total -- will need food aid up to March, with fears of localised famine if combatants continue to shell food distributions, loot aid and menace humanitarian staff.

Conflict erupted in December 2013 between President Salva Kiir and his former deputy, Riek Machar, forcing two million people to flee their homes and killing up to 50,000.

"It was extremely painful to see," the U.N.'s emergency relief coordinator Valerie Amos said at a news conference in Nairobi, describing her three-day visit to the world's youngest nation where she met displaced people who were exhausted, having fled numerous times.

"If this conflict continues, we will lose a generation of South Sudan's children... I have seen the blankness in their eyes because they cannot actually afford to remember what they have seen."

On Monday, donors at a pledging conference committed $529 million, which, added to previous funds, provides $618 million to repair roads and airstrips and pre-position supplies before the rains come in May, when access will be much harder.

The United Nations has appealed for $1.8 billion to fund the emergency response in South Sudan in 2015.

South Sudan is the scene of the world's largest aid operation. Almost 20,000 aid workers belonging to 120 agencies delivered $1.4 billion of aid to 3.6 million people in over 200 locations in 2014, the U.N. said.

Amos called on the warring parties to step up their efforts to find peace. According to the latest roadmap, peace talks will resume on Feb. 19 and be completed by March 5.

"The government and the opposition need to think about their people," she said.

"If they actually went out and saw the conditions in which their people are living, it might actually mean that they would pause and think again about the importance of peace." (Reporting by Katy Migiro; Editing by Ros Russell)