30 April 2014 Last updated at 11:04George Clooney has visited Sudan and South Sudan on several occasions
Actor George Clooney has ended his role as a United Nations "Messenger of Peace" after six years.
The Hollywood star used the position to help raise awareness of conflicts in countries like Sudan and the Democratic Republic of Congo.
UN spokesman Stephane Dujarric said the Hollywood star "feels it is time to retire his official role".
Clooney has campaigned about the Darfur crisis and violence in South Sudan, which gained independence in 2011.
By James CopnallSudan analyst and author
As an activist, George Clooney is most known for campaigning about Sudan and South Sudan - but this has largely taken place independently from the UN.
His most prominent engagement has been with the Darfur civil war in western Sudan, though he has also travelled into another Sudanese conflict area, South Kordofan, and on advocacy trips to South Sudan before and after its 2011 independence.
His presence has generated interest in the Sudans.
But he and the Enough Project are accused of distorting complex issues, and even of making it harder to find a solution to the Sudans' many problems.
James Copnall is the author of A Poisonous Thorn in Our Hearts: Sudan and South Sudan's Bitter and Incomplete Divorce
Two million people have been displaced by the conflict in Darfur, western Sudan, since a civil war began in 2003. Meanwhile, South Sudan descended into violence in December - just over two years after it became the world's newest nation.
As well as his UN role, Clooney joined fellow actors including Matt Damon and Brad Pitt to form the charity Not On Our Watch, which aims to "bring global attention to international crises and give voice to their victims".
He also has close ties with campaign group the Enough Project and helped found the Satellite Sentinel Project, which uses spy satellites to track human rights abuses and atrocities in Sudan.
The UN says its Messengers of Peace are high-profile people who raise awareness of the organisation's work around the world.
"The competing demands on their time from their professional and advocacy lives sometimes make it difficult for high-profile individuals to carry out a formal United Nations role," Mr Dujarric said.
The spokesman added that Clooney's move would allow him the independence to pursue "other personal advocacy projects and activities".
Other UN Messengers of Peace include actors Michael Douglas and Charlize Theron, musicians Lang Lang and Stevie Wonder, and author Paulo Coelho.