By Ben Westcott, Jennifer Hansler and Sarah Faidell, CNN

(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."

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."

According to the New York Times, who first reported the story on Sunday, Victor Liu was due to start his second year at Georgetown University in Washington before their trip to China, while his sister Cynthia had a job lined up in New York.

Their mother, Sandra Han, was detained shortly after her children's arrival in June. When the siblings arrived at the airport, they were told by authorities they weren't allowed to leave. Their current location in China is unclear.

Their father Liu Changming is accused by Beijing of issuing billions of dollars in illegal loans before fleeing the country in December 2007. In 2009, China issued a worldwide Interpol red notice for his arrest and he appeared on a list of China's 100 top fugitives in 2015.

In a statement provided to CNN, the Liu family's lawyer, Washington-based David Pressmanhe, said: "Out of concern for the security of these young Americans, we will refrain from public comment as we continue our efforts to constructively and directly engage the Chinese government to allow them to return home."

The controversy comes ahead of US President Donald Trump's planned meeting with Chinese leader Xi Jinping at the G20 summit in Argentina this week, where the two leaders will discuss a possible end to the US-China trade war.

Representatives for the Liu family are hopeful Trump will raise their case with Xi. The US State Department official told CNN they were in "close contact" with Victor and Cynthia Liu and were providing consular assistance as appropriate.

"We routinely have shared our concern about China's use of exit bans on American citizens with the Chinese government and will continue to do so until we see a transparent and fair process," the official said.

China has a long history of imposing exit bans on individuals in an attempt to force suspected criminals to surrender, or to resolve business disputes.

In January, the US State Department issued a travel warning to US citizens, noting how foreign nationals had been "arbitrarily detained or detained for reasons related to 'state security'."

Georgetown University President John DeGioia met with Liu in Beijing earlier this month. In a statement to CNN, the university said it was working with representatives in Beijing on behalf of its student.

Speaking at a press conference on Monday, Chinese Ministry of Foreign Affairs spokesman Geng Shuang said all members of the Liu family have "valid Chinese citizenship identity documents" and were subject to Chinese laws.

"They are suspected and involved in financial crimes and restricted from leaving the country according to law. I want to repeat that China is a lawful country. Chinese judicial authorities will deal with the case according to law," he told reporters.

The Chinese government doesn't recognize dual citizenship of its citizens, instead considering all dual Chinese citizens and their children to be Chinese nationals.

Since the two young Americans arrived in June, they've attempted to leave three times. The two claim their father severed ties with the family in 2012, according to the New York Times.