US authorities have announced charges against 80 people, most of them Nigerians, in a wide-ranging fraud and money laundering operation that netted millions of dollars from victims of internet con jobs.

American authorities on Thursday unsealed a 252-count grand jury indictment charging those people in an operation that procured at least $46m from victims of internet scam jobs.

Seventeen people were arrested and taken into custody in Los Angeles and other cities in the United States.

The investigation began in 2016 with a single bank account and one victim, said Paul Delacourt, assistant director in charge of the Federal Bureau of Investigation's (FBI) Los Angeles office.

The suspects used a variety of scams to allegedly target the elderly, individuals seeking romantic relationships, and small and large businesses, to convince them to send money online.

"We believe this is one of the largest cases of its kind in US history," US Attorney Nick Hanna told a news conference. "We are taking a major step to disrupt these criminal networks."

The indictment alleges that at least $6m in fraudulently obtained funds were transferred through a money-laundering network run by two Nigerians - Valentine Iro, 33, and Chukwudi Christogunus Igbokwe, 38.

The pair were among those arrested.

'Extremely depressed'

One of the victims, identified as only "F.K." in court papers, was reportedly "extremely depressed and angry about these losses", the federal complaint stated.

"She began crying when discussing the way that these losses have affected her."

F.K., a Japanese woman, started an online relationship with a fraudster named Terry Garcia, claiming to be stationed in Syria, through an international social network for digital pen pals.

The two would communicate over email, with her using Google to translate his English into her Japanese.

Garcia told her he had found a bag of diamonds in Syria within a month of their relationship, and began introducing her to his associates, starting with a Red Cross representative who told her Garcia had been injured but had given him the box.

In total, she made 35 to 40 payments, receiving as many as 10 to 15 emails a day directing her to send money to accounts in the US, Turkey and the United Kingdom through Garcia's many purported associates.

At one point, she was even threatened with arrest if she did not make the payments.

The relationship ended with F.K. losing $200 000 and on the verge of bankruptcy. She had borrowed money from her sister, ex-husband and friends to help Garcia with his fraudulent plan.

All the defendants face charges of conspiracy to commit fraud, conspiracy to launder money, and aggravated identity theft, while some are charged with additional offences.

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