The US Consulate in Barcelona warned American citizens on Wednesday via its website about the increase in robberies and violent crime in Barcelona so far this summer.
It advised people to take extra care when visiting the most popular tourist areas of the Catalan capital, where thefts of watches, jewellery and wallets have become more common.
It added that Catalan authorities have "indicated that they are seeking to address these issues".
Police data shows that since 2016 robbery with violence or intimidation has increased 58 per cent in Barcelona and patrols have been stepped up.
Foreign media are also starting to report on the city's rise in crime, worrying tourist bosses.