Antonio Maria Costa, Executive Director of the UN Office on Drugs and Crime, said: "The anti-narcotics police in Iran are among the best in the world."

During his recent visit to Tehran, Costa met the Minister of Foreign Affairs, the Secretary-General of the Drug Control Headquarters and the Minister of Health, and praised the Iranian Government which he said was making "a massive sacrifice" in its efforts against drug smuggling.

"Iran was holding back a flood of heroin from Afghanistan," the UN official said.

According to UNODC estimates, most of the opium going from Afghanistan to the West is smuggled via Iran. This amounts to approximately 2,500 tons of opium crossing Iran's borders every year. Costa said the Iranian authorities seized about one-third of that amount.

"Most of the world's opium is produced in one country (Afghanistan). The more drugs that are seized near production areas, the less drugs will reach Western streets," he said, adding "this should be a shared responsibility, not only Iran's problem."

Costa visited a memorial to the more than 3,700 border control officers who have died while fighting well-armed drug traffickers. More than 12,000 others have been injured.

"These fallen police officers have given their lives to prevent drugs from poisoning the world, not just Iran," Costa said.

He also visited a drug rehabilitation centre, and spoke with addicts attempting to shake their habits.

"Iran has a major drugs problem, but it is taking the right steps to deal with it," Costa said.