George Clooney addressed the UN Security Council in 2006

The Hollywood star has campaigned to stop the conflict in Darfur, Sudan.

UN Secretary General Ban Ki-moon hailed his "dedication to raising awareness and mobilising action on Darfur".

Clooney pledged to build support for the UN's "critically important work in some of the most difficult, dangerous and dire places in the world".

"I am deeply honoured to receive this appointment," he said.

In 2006, Clooney delivered an impassioned speech to the UN Security Council, telling its members that genocide was taking place "on your watch".

'Personal mission'

Last April, the Ocean's 11 and Syriana star set up a charity, Not On Our Watch, with fellow actors Don Cheadle, Brad Pitt and Matt Damon to raise funds for Darfur.

The charity has so far gathered more than $9.3m (£4.75m) for humanitarian work in the area.

At least 200,000 people have been killed in Darfur and two million forced from their homes in the five-year conflict between the government and rebel groups.

The UN recently took control of the peacekeeping mission in the region, although the force is still far short of its intended total of 26,000 troops.

Mr Ban told Clooney, who has been to Darfur: "You have seen first-hand the pain experienced by the victims of war and made it your personal mission to help end violence and human suffering."

Clooney joins eight other current UN messengers of peace, who include conductor Daniel Barenboim, author Paulo Coelho, cellist Yo-Yo Ma and film star Michael Douglas.