ROME (Reuters) - Italian police said on Wednesday they had seized assets worth 2 billion euros ($2.2 billion) and issued 41 arrest warrants in raids on gambling companies in Italy and abroad run by the 'Ndrangheta mafia organization.

Police believe the firms, including six that were operating out of Malta, were used to launder vast sums of illicit cash.

Italy's mafia groups, including the 'Ndrangheta from the southern region of Calabria, the Camorra from Naples and Sicily's Cosa Nostra, have strengthened their grip on southern Italy's economy and spread to the richer north during a three-year economic slump.

Among the targets of Wednesday's operation were 1,500 betting shops, 82 gambling websites, 45 Italian companies and 11 foreign firms, as well as "innumerable" property assets, the financial police said in a statement.

Apart from the firms run from Malta, two were in Spain, two in Romania and one in Austria.

The Calabrian mafia has grown rich on the drugs trade, becoming one of Europe's biggest importers of South American cocaine, with strong ties to Mexican and Colombian cartels.

It also runs companies in sectors including trucking and hospitality, with a stranglehold on the economy of Calabria, where unemployment is above 20 percent.

($1 = 0.9146 euros)

(Reporting by Isla Binnie; Editing by Andrew Bolton and Raissa Kasolowsky)