Hong Kong (CNN) Ahead of a high stakes meeting between Chinese and US leaders at the G20 summit in Buenos Aires, a top US security official has demanded the release of two young Americans who say they've been prevented from leaving China.

Victor and Cynthia Liu, aged 19 and 27, and their mother, Sandra Han, are believed to have been refused permission by Beijing authorities to leave China, where they have been held since June.

US authorities told CNN they are working closely with the two Liu children who are reportedly being used to pressure their father, Liu Changming, a high-profile Chinese fugitive, into returning to Beijing where he is wanted for financial crimes.

On Tuesday, US National Security Adviser John Bolton became the most high-profile US figure to comment the case, with a post on Twitter declaring: "These Americans need to be allowed to return home."

Victor and Cynthia Liu

Even though Victor Liu was born in the US and both Liu children hold a US passport, Beijing views them as citizens of China and subject to its jurisdiction. China has accused the Liu family of being "involved in economic crimes."