George Clooney's mum and dad watch as he becomes UN messenger of peace

But Clooney was blocked from delivering a message on his recent trip to Darfur in western Sudan, Chad and the Democratic Republic of Congo to a meeting of nations contributing peacekeeping troops.

Diplomats said several countries including Russia had objected to his presence as inappropriate.

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Instead, Clooney delivered the message to a news conference.

"The message is the world is watching and that at this point we cannot afford to fail," he said.

"Millions are homeless, not from famine or disease or acts of God, but from a well armed militia intent on ridding the land of its people."

With fellow actors Don Cheadle and Brad Pitt, Clooney,46, has used his celebrity status to raise money for refugees through their "Not On Our Watch" charity and draw attention to the crisis in Darfur.

The United Nations is trying to deploy a peacekeeping force in Darfur, where experts say some 200,000 people have been killed and more than 2 million driven from their homes in fighting between Sudan's government and rebels.

"We tend to not get to see enough of what we need to see anymore," said Clooney, who last month was honored along with Cheadle by Nobel peace laureates for their Darfur efforts.

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"It seems as if at times celebrity can bring that focus. It can't make the policies, it can't change people's minds really, but you can bring a camera where you go because they'll follow you and you can shine a light on it. That seems to be my job."

Clooney is the ninth messenger of peace - people chosen from art, music, literature and sports who agreed to help focus attention on U.N. work. The program was started in 1998 by U.N. Secretary-General Ban Ki-moon's predecessor, Kofi Annan.

The actor, accompanied by his parents, was greeted at U.N. headquarters by hordes of squealing employees.

Clooney said his new role was to raise awareness about U.N. peacekeeping efforts generally and he would "go wherever I have to to bring some attention."

He also plans to lobby countries to fulfill their U.N. peacekeeping funding commitments, specifically mentioning the United States, which he said owed $1.2 billion.

"These men and women risking their lives for peace are your responsibility. So either give them the basic tools for protecting the population and themselves or have the decency to bring them all home because you can't do it halfway," he said.

Clooney, who won an Oscar in 2005 for his role in "Syriana" and has been nominated this year for his role in "Michael Clayton," recalled how desperate people in Darfur had pleaded to him for U.N. help.

"When I stood in the hospital next to women who had been raped and set on fire they looked up to me and said 'please send the U.N.,' not the U.S., not China, not Russia, just the U.N. You are their only hope," he said.

The other U.N. messengers of peace are actor Michael Douglas, musicians Daniel Barenboim, Midori Goto and Yo-Yo Ma, authors Paulo Coelho and Elie Wiesel, naturalist Jane Goodall and Olympic equestrian Princess Haya of Jordan.