By Peter Apps

LONDON, August 14 (Reuters) - Lobby groups must ensure they get their facts straight, Britain's advertising watchdog has warned after finding that the Save Darfur Coalition could not substantiate its claim of 400,000 deaths in the Sudanese region.

Made up from dozens of faith-based and advocacy organisations and backed by celebrities such as Mia Farrow and George Clooney, the US-based Save Darfur Coalition has been credited with drawing international attention to the situation in Sudan's war-torn west.

But Britain's Advertising Standards Authority said in a ruling last week that recent adverts saying that 400,000 people had died in violence that the coalition blames on the Sudanese government went too far.

"There is no dispute that atrocities are going on there," said ASA spokesman Matt Wilson. "One doesn't want to quibble when it comes to mass slaughter but they simply could not substantiate the figure of 400,000."

The Sudanese government says only 9,000 people have died since rebels took up arms in 2003 but most international experts put the figure at around 200,000.

Some 2.5 million people have fled their homes and the U.S. government accuses Sudan of genocide -- a charge the Sudanese government denies.

The ASA made its ruling in response to a complaint by a group called the European Sudanese Public Affairs Council, whose Web site says it exists in part to challenge questionable coverage of Sudanese affairs.

Aid and human rights workers say they are often under pressure to come up with headline-grabbing figures in emergencies -- but that counting the dead on the ground can be practically impossible.

The ASA says advocacy groups should be careful in their claims.

"We do try to be more sensitive when it comes to charities but there are limits," said Wilson. "We would encourage them to make sure that any figures or facts they use in adverts are accurate."

The Save Darfur Coalition said it accepted the ruling -- which said that the advert should have made it clear that 400,000 was at best an upper end estimate rather than an established fact -- but still believed its figures reflected the true size of the slaughter.

"History tells us that the size of genocides are rarely understood contemporarily," it said in a statement.

"The government of Sudan is actively obstructing any accurate verification at this time and the international community must continue to pressure President Omar al-Bashir to provide access to international peacekeepers and humanitarian workers who can more accurately document the scale of this genocide."

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