The relationship between a Japanese woman and a US army captain stationed in Syria started online, through an international social network for digital pen pals, but soon grew into an online romance that spanned over 10 months of daily emails.

Key points: The FBI has charged 80 mostly Nigerian nationals in a $US46-million case

The FBI has charged 80 mostly Nigerian nationals in a $US46-million case The alleged criminal network targeted the elderly and people susceptible to romance scams

The alleged criminal network targeted the elderly and people susceptible to romance scams 14 defendants were arrested in LA on Thursday while others were still on the run

It ended with the woman $US200,000 ($296,000) poorer and on the verge of bankruptcy after borrowing money from her sister, ex-husband and friends to help captain "Terry Garcia" with his plan to smuggle diamonds out of Syria.

In reality, there were no diamonds and there was no Garcia — they were part of an elaborate scam hatched by an international ring of cyber thieves operating mainly out of Los Angeles and Nigeria.

Federal authorities cited the case of the Japanese woman, known only as "FK" in court papers, when they announced an indictment charging 80 mostly Nigerian defendants with stealing $8.8 million and attempting to steal another $59 million through various schemes that targeted businesses, the elderly and anyone susceptible to a romance scam.

More than 80 people — mostly Nigerian nationals — have been charged by the FBI in one of the biggest 'love scam' cases in US history, according to the US government. ( AP: Reed Saxton )

"We believe this is one of the largest cases of its kind in US history," US Attorney Nick Hanna said.

"We are taking a major step to disrupt these criminal networks."

The arrests come almost four months since Australian woman Yoshe Taylor was released from a Cambodian prison after she was lured into an internet love scam by a Nigerian man, Precious Chineme Nwoko, who was living in Phnom Penh.

The Brisbane mother had already served six years of her 23-year sentence when a Cambodian court ruled she was an innocent victim of a drug-smuggling operation.

Yoshe Taylor (centre left), Precious Chineme Nwoko and Charlene Savarino were arrested at Phnom Penh airport in late 2013. ( Supplied )

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

It grew to encompass victims who were targeted in the US and around the world, some of whom like the Japanese woman lost hundreds of thousands of dollars.

"FK was and is extremely depressed and angry about these losses," the federal complaint states.

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

Her relationship began innocently in March 2016 with an email but soon "Garcia" made "romantic overtures," according to federal authorities.

He told her they couldn't talk by phone because he wasn't allowed to use one in Syria.

So a stream of emails went back and worth, with her using Google to translate his English into her Japanese.

A month into the relationship, Garcia told her he had found a bag of diamonds in Syria and he 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.

FK ultimately 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 the captain's many purported associates.

The fraudsters even threatened her with arrest if she did not continue to pay and at one point she travelled to Los Angeles because she was told a Russian bank manager had embezzled more than $49,000 of her funds.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 16 seconds 1 m 16 s Yoshe Taylor had a huge task ahead of her to prove her innocence.

Authorities arrested 14 defendants on Thursday (local time), mostly in the Los Angeles area.

FBI agents could be seen processing suspects in a downtown Los Angeles parking lot before they were arraigned in federal court.

Two suspects were previously in custody and a few others were arrested earlier this week.

Mr Hanna said he hoped to be able to work with foreign governments to extradite the remaining defendants.

They all face charges of conspiracy to commit fraud, conspiracy to launder money, and aggravated identity theft, and some are charged with additional offenses for alleged fraud and money laundering.

AP