Marijuana use worldwide is out of control because the plant grows everywhere, is in high demand and erroneously is considered by many to be harmless, a U.N. agency said Monday.

Antonio Maria Costa, executive director of the United Nations Office on Drugs and Crime, highlighted the marijuana problem at a news conference in releasing his office's 2006 World Drug Report.

Marijuana, also known as cannabis, is a danger because it contains THC, a banned substance, Costa said.

While governments around the world generally have succeeded in containing use of heroin, cocaine and amphetamines, marijuana is a different story, Costa said.

"It's out of control in supply because it's a weed; it grows everywhere. It's out of control in demand because it's erroneously considered a light drug," he said. "But, and indeed, it is extremely problematic because of much-increased THC, tetrahydrocannabinol, content."

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John Walters, head of the White House Office of National Drug Control Policy, agreed that marijuana is "a massive global problem."

"It's not just a gateway, it is a dead end as well as an opening for many other people who go on and use other things, and are polydrug users. It has been for a long time," said Walters, who joined Costa at the news conference.

The U.N. report released by Costa generally was upbeat, boasting gains including that opium poppy cultivation was down 22 percent in 2005.

On the negative side, the report found that cocaine use is reaching alarming levels in Western Europe.

It attributed the decline in opium poppy cultivation to cutbacks in the three main source countries of illicit opium in the world: Afghanistan, Myanmar and Laos. Opium is the main ingredient for heroin.

"In Afghanistan, in 2005, opium poppy cultivation decreased for the first time since 2001," the report said. Still, it said, that country accounted for 89 percent of opium production worldwide.

The State Department's annual report on illicit drugs, issued in March, acknowledged that opium production is hampering democracy-building efforts in Afghanistan.

"Afghanistan's huge drug trade severely impacts efforts to rebuild the economy, develop a strong democratic government based on rule of law, and threatens regional stability," the report said.

Despite the sharp decrease in the total area under cultivation in all opium-producing countries, the U.N. report said production was down only 5 percent due to more favorable weather conditions during the 2005 growing season in Afghanistan.