The US authorities say they have frozen the assets of six Nigerian nationals for cyber fraud amounting to about $6m (£4.7m).
The fraudsters targeted American individuals and small businesses in an “elaborate scheme’’ of romantic fraud or by impersonating business executives.
They allegedly received illegal money transfers and manipulated their victims to access their usernames, password and bank details.
Some victims of romance scams were targeted through emails and social media.
The US authorities say the majority of those affected were women, the elderly and those who had lost a spouse.
The whereabouts of the suspects is unknown, but authorities say all their property and interests in the US have been blocked. How much this amounts to is unclear.
American citizens have also been prohibited from dealing with the fraudsters.
Secretary of State Mike Pompeo said the fraudsters had taken advantage of technological advancements to target vulnerable Americans.
In August 2019, the US charged 80 foreign nationals, mostly Nigerians, for the attempted theft of $46m through internet fraud.
Last year, the US Treasury Crimes Enforcement Network said it had received more than 32,000 reports involving almost $9bn in attempted theft schemes targeting US financial institutions and their customers.