* U.N. world drugs report paints mixed narcotics picture
* Afghan opium cultivation surges; cocaine production down
* Illicit drugs kill up to 200,000 every year - UNODC chief
VIENNA, June 26 (Reuters) - More Americans are consuming marijuana as their perception of the health risks declines but more are seeking help for problems related to the drug, a U.N. report said on Thursday.
The U.N. Office on Drugs and Crime (UNODC) said it was still too early to understand the impact of recent cannabis legalisation moves in the U.S. states of Washington and Colorado and the South American country of Uruguay.
However, research suggests that declining risk perception and increased availability can lead to wider use and to more young people being introduced to the drug, the UNODC said in its annual report on the narcotics situation in the world.
Global cannabis use seemed to have decreased, the report said, reflecting a decline in some western and central European countries.
"However, in the United States, the lower perceived risk of cannabis use has led to an increase in its use," UNODC said, without specifying what may have caused this change.
Washington and Colorado have legalised the sale of cannabis under license, but U.S. federal laws still prohibit sales.
The report said the number of people in the country aged 12 or more who used cannabis at least once in the previous year rose to 12.1 percent in 2012 from 10.3 percent in 2008.
But more people are seeking treatment for "cannabis-related disorders" in most regions of the world, including in North America.
This is one reason that expected tax revenue from retail cannabis sales should "be cautiously weighed against the costs of prevention and health care," the 2014 World Drug Report said.
In December, Uruguay's Congress approved a law allowing the cultivation and sale of marijuana, making it the first country to do so, with the aim of wresting the business from criminals.
The experiment is being keenly watched by Latin American peers at a time when the U.S.-led war on drugs faces mounting criticism. Success in Uruguay could fuel momentum for legalisation elsewhere.
Regarding other narcotics, a surge in opium production in Afghanistan - where the area under cultivation jumped by 36 percent in 2013 - was "a setback" while the global availability of cocaine fell as production declined from 2007 to 2012.
Last year, the worldwide output of heroin "rebounded to the high levels witnessed" in 2008 and 2011, UNODC added.
"Up to 200,000 people die every year due to illicit drugs," UNODC executive director Yury Fedotov said in a statement. (Reporting by Fredrik Dahl; Editing by Raissa Kasolowsky)