Anastasia Vashukevich, known by her pen name Nastya Rybka, and self-styled sex guru Alexander Kirillov have been detained since they and eight other foreigners were arrested in February by Thai police who raided their "sex training course" in the seaside city of Pattaya.
They were initially charged with lacking a work permit but are now facing additional charges of soliciting prostitution and criminal association, according to Apichai Krobpetch, Pattaya's police chief.
The pair, who are embroiled in a political scandal back in Russia, made international headlines after Vashukevich offered to reveal secrets to American journalists in a video posted on Instagram shortly after their arrest in Pattaya.
"They are trying to put us behind bars... That is why I am ready to tell you about all those missing puzzle pieces that you lacked... regarding a link between our esteemed lawmakers and (Paul) Manafort, Trump and all this brouhaha, the US elections," she said in the video.
The model, who has written a book about seducing oligarchs, has not substantiated her claims but does have links to Russia's elite.
She and Kirillov are facing a lawsuit in Russia over footage which Vashukevich filmed purporting to show an influential deputy prime minister, Sergei Prikhodko, enjoying lavish hospitality on a yacht owned by billionaire Oleg Deripaska.
The video went viral after it was published by top Kremlin critic Alexei Navalny in February.
Deripaska, one of several Russian oligarchs sanctioned by the US this month, denied any wrongdoing and later sued the pair for invasion of privacy.
"We will not go back to Russia because they opened a new criminal case for us," Vashukevich told AFP from a police van on Tuesday before she and other defendants were whisked into the Pattaya courtroom for the plea hearing.
Asked if he had a message for the US, Kirillov responded: "Help us anyway, because we don't know what is happening."
The US embassy in Bangkok has repeatedly declined to comment on the case.
Deripaska, an aluminium tycoon whom Washington has accused of operating for the Russian government, was once an associate of US President Donald Trump's ex-campaign manager Paul Manafort.
Manafort has been indicted on money-laundering and tax-related charges as part of the special counsel's investigation into Russian interference in the 2016 US election.
The Russian embassy did not immediately respond to requests for comment on Tuesday.
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