(Adds comment from U.N. aid official)
UNITED NATIONS, July 23 (Reuters) - U.N. Secretary-General Ban Ki-moon on Wednesday named Danish diplomat Ellen Margrethe Loj as his new special envoy to South Sudan and head of the U.N. peacekeeping mission in the world's newest nation.
Loj replaces Hilde Johnson of Norway, who stood down this month after three years in the job, amid renewed violence sparked by a power struggle between President Salva Kiir and his sacked deputy Riek Machar.
The conflict has reopened deep ethnic tensions in the South Sudan, which only won independence from Sudan in 2011, pitting Kiir's Dinka against Machar's Nuer. Thousands of civilians have been killed and more than a million forced to flee their homes since December, prompting U.N. warnings of a looming famine.
Ban said Loj "brings to the position a wealth of experience in peacekeeping and international affairs, having served as the Special Representative of the Secretary-General to the United Nations Mission in Liberia from 2008 to 2012."
She has also been Denmark's ambassador to the Czech Republic, the United Nations and Israel.
Senior U.N. humanitarian official John Ging described the situation in South Sudan on Wednesday as a "mega-crisis" that is deteriorating at an alarming rate.
"Over 3.8 million people inside South Sudan need humanitarian assistance and this number continues to grow. We have on the horizon the prospects even of famine," Ging told reporters at the United Nations.
"We are preparing for the worst but we are not accepting that it has to unfold this way," Ging said. "It is going to be a catastrophe of quite unprecedented status." (Reporting by Michelle Nichols and Mirjam Donath; Editing by James Dalgleish)