The top UN official for humanitarian aid, Valerie Amos, who oversaw international relief efforts in Syria and other trouble spots, announced Wednesday, Nov 26, she is stepping down, according to Agence France-Presse.

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. 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."

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.