One of two Syrian sisters who jumped into the sea and pulled a sinking boat to safety in 2015 has been arrested after the aid group she volunteered for was accused by Greece of illegally smuggling people into the country, her lawyer said on Friday.

Sarah Mardini, 23, and her sister Yusra, an Olympic swimmer, saved 19 refugees by swimming the boat to the Greek island of Lesbos in 2015, but she has now been placed in a maximum security prison.

Greek authorities have accused the Greek non-profit Emergency Response Centre International (ECRI) of smuggling migrants into the country, spying and money laundering.

"She was strictly doing volunteer work. There is not a piece of evidence against her," said Mardini's lawyer Haris Petsikos.

Authorities said the suspected crime gang allegedly "provided direct assistance to organised migrant trafficking rings" but the organisation has denied the allegations.

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"We are certain that many of the unfounded claims, accusations and charges will be quashed, allowing the truth to prevail and justice to be served," the organisation said in a statement.

Mardini had settled in Germany but spent time in Greece as a volunteer where the organisation provides help for refugees living on Lesbos.

A date for a trial has not yet been set. Under Greek law, Mardini could be held for up to 18 months.

In May, Greece cleared five European volunteers who regularly saved sinking boats but were arrested in 2016 and charged with human smuggling.

Mardini and her younger sister Yusra, a UNHCR goodwill ambassador and swimmer who competed in the 2016 Rio Olympics with the first ever refugee team, fled Damascus in 2015 for Turkey.

They boarded an overcrowded dinghy to Greece and, when it began taking on water, jumped into the sea and dragged it for hours to Lesbos, saving the lives of 19 others.

Nearly a million Syrians, Afghans and Iraqis crossed to Lesbos from Turkey in 2015, at the height of Europe's refugee crisis, before travelling onwards to northern Europe. Dozens of aid groups operated on Lesbos at the time.