VIENNA (Reuters) - Afghan opium production fell last year for the first time since Taliban rulers were toppled in 2001 but could surge again this year due to persistent lawlessness around the country, a UN report said on Monday.

Taliban insurgent attacks are increasing, helping drug gangs exploit insecurity, weak policing, rampant corruption and a reluctance to pursue powerful figures involved in drugs but supporting the U.S.-led war to stamp out the Taliban.

"Opium production in Afghanistan could rise again this year despite a welcome decline in 2005," the annual report by the Vienna-based UN Office on Drugs and Crime (UNODOC) said.

Poppy cultivation last year declined 21 percent in Afghanistan due to state efforts to destroy it and encourage farmers to grow alternative crops. Afghanistan produces 90 percent of the world's opium and its refined form, heroin.

"But Afghanistan's drug situation remains vulnerable to reversal because of mass poverty, lack of security and the fact the authorities have inadequate control over its territory," UNODOC chief Antonio Maria Costa said in the report. "This could happen as early as 2006 despite large-scale eradication of opium crops this spring."

The report cited three key weaknesses in global drug control efforts -- heroin supply from Afghanistan, cocaine demand in Europe and cannabis supply and demand everywhere.

"Too many professional, educated Europeans use cocaine, often denying their addiction, and drug abuse by celebrities often presented uncritically by the media, leaving young people confused and vulnerable," it said.

Cannabis was used by an estimated 162 million people at least once in 2004, roughly four percent of the global population aged 15 to 64, and consumption continues to go up.

"Traffickers have invested heavily in increasing the potency, and therefore the market attractiveness, of cannabis, the report said. "As crime cartels look for new trafficking routes, especially towards the European Union, countries in the Caribbean, West Africa and Central Africa are under attack."

Afghan opium production ballooned after the Taliban, who banned opium at the end of their rule, were ousted.

UN anti-drugs experts believe the answer to Afghanistan's drug problem is providing farmers with moneymaking options. But that means developing the country's war-ruined rural economy, and that will take years.

The UN drugs body cited one success over drug control in Laos. The poor Southeast Asian state was the world's No. 3 opium provider until the mid-1990s, but slashed poppy cultivation by 72 percent in 2005 and is now almost opium-free, it said.