LONDON (Reuters) - Iranian authorities have seized more than six tonnes of heroin destined for Europe and dismantled five international gangs who were trafficking the drugs, state media reported on Tuesday.

European nationals were among 11 members of one of the main gangs who were arrested, Iran’s intelligence ministry said.

A ministry statement carried on state news agency IRNA did not specify the nationality of the detainees or the date of the operations.

Drug trafficking is a serious challenge for the Islamic Republic as it borders Afghanistan - the world’s largest opium producer - and Pakistan, a major transit country for drugs.

Iran has one of the strongest counter-narcotics forces in the region. In 2012, it accounted for two third of the world’s opium seizures and one fourth of the world’s heroin and morphine seizures, according to a U.N. report published in 2014.

Italian police last week found 270 kg (600 lb) of heroin hidden in a container that arrived on a ship from Iran, the biggest such haul in Italy for at least 20 years. The freighter sailed from Bandar Abbas port in Iran’s south.

Iran has one of the world’s most severe addiction problems. Deputy police chief Eskandar Momeni said on Monday there were 2.8 million drug users in Iran.