The deputy to the United Nations' Syria mediator Lakhdar Brahimi will leave his post this week, the UN said on Monday.

The UN did not specify the reason for Nasser al-Kidwa's prompt departure, but said that he had indicated "his willingness to serve the United Nations in other capacities."

UN chief Ban Ki-moon thanked Kidwa for his efforts "in trying to end the bloodshed in Syria and move toward a Syrian-led transition," according to a statement by Ban's spokesman Martin Nesirky.

The nephew of former Palestinian leader Yasser Arafat, Kidwa was nominated to the job in August 2012 at the same time as Brahimi, having served his predecessor Kofi Annan as well.

Kidwa was in charge of contact with the Syrian opposition, in particular, but diplomats say that he was criticized by the government in Damascus, which repeatedly denied him a visa.

Ban "expressed his appreciation" to Kidwa for his service under both Brahimi and Annan, the statement said

Asked if Kidwa will be replaced, Nesirky said: "Discussions are still going on about that... Stay tuned."