Pope Francis spoke out on Wednesday against the legalisation of drugs and denounced drug traffickers as "assassins", while also calling for help and support for addicts.

"A reduction in drug addiction is not achieved by liberalising drug use - this is an illusion - as has been proposed, or already implemented, in some countries," Francis told crowds in St Peter's Square at his weekly audience.

"At the same time, however, let us remember that each addict has a unique personal story and must be listened to, understood, loved, and, insofar as possible, healed and purified," he said on what the United Nations has designated as World Drug Day.

Francis, 87, drew no distinction between so-called soft drugs such as marijuana, which has been legalised for recreational use in several countries and some US states, and "hard" ones such as heroin and cocaine.

"We cannot ignore the evil intentions and actions of drug dealers and traffickers. They are assassins," the pope added.

Francis, who is from Argentina, also condemned the "destructive impact" of drug production on the environment, saying it was "increasingly evident, for example, in the Amazon basin" in Latin America.