The top UN official for humanitarian aid, Valerie Amos, who oversaw international relief efforts in Syria and other trouble spots, announced Wednesday she is stepping down.
Amos had been in the key post for more than four years, overseeing major aid operations in Syria as well as in South Sudan, Iraq and the Central African Republic.
Her departure comes as the United Nations is struggling to cope with a record 50 million people displaced from conflict.
In a letter to UN staff, Amos said she would be leaving in March after working at "the most challenging, demanding and rewarding job I have ever done" as UN under-secretary general for humanitarian affairs and emergency relief coordinator.
She gave no specific reason for her departure but recalled that she was now the longest-serving top UN aid official, one of the most demanding jobs at the United Nations.
A UN official close to Amos told AFP "she feels it's time to move on" after having been in the position since August 2010.
UN Secretary General Ban Ki-moon expressed gratitude and praised Amos for the extensive experience she brought to the post.
She "has helped find solutions for people who are facing the worst experiences of their lives," he said in a statement.
Ban said the 60-year-old former British minister for international development "has tirelessly advocated for people around the world affected by disaster and conflict."
"For her, people have always come first," he said.
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- Focus on aid to Syria -
The announcement came the day after Amos reported to the UN Security Council on humanitarian aid efforts in Syria, which has been torn by a brutal war for nearly four years.
Amos appealed for more cross-border aid deliveries and urged the 15-member council to push Damascus and all sides toward a political solution to end a war that has been a focal point of her tenure.
There are more than 7.6 million people displaced inside Syria and 3.2 million others have fled the country -- the largest number of people displaced from conflict in the world.
"I have seen the very worst of what we as human beings can do to each other but I have also seen the very best," Amos wrote in her letter.
She paid tribute to "people with almost nothing" who are willing to share what little they have and to humanitarian workers who risk their lives to serve people in need.
Born in Guyana, Amos has been a lifelong campaigner for human rights.
She was a minister for Africa from 2001 to 2003, in the government of former prime minister Tony Blair, and was chair of the Royal African Society from 2007 to 2009.
She also served as president of the privy council, leader of the House of Lords and British high commissioner to Australia.
Amos, who holds the title of baroness, was the first black woman to sit in the British cabinet following her appointment as international development secretary in 2003.