HONG KONG (REUTERS) - More than 150 parliamentarians from over 18 countries have called on Hong Kong leader Carrie Lam to intervene to ensure justice for 12 people, the youngest of whom is aged 16, who have been detained in mainland China while trying to flee the city by boat.

The 12, who had all faced charges in Hong Kong linked to anti-government protests, have been held virtually incommunicado in a mainland prison since they were detained at sea on Aug 23, apparently while trying to reach the democratic island of Taiwan.

Chinese authorities said last week that members of the group face charges of illegal border crossing and organising an illicit border crossing, which could carry a sentence of up to seven years in jail.

"In your role as Chief Executive, it is incumbent on you to intercede on behalf of these young people to ensure that they are guaranteed justice," the parliamentarians said in the letter released late on Tuesday (Dec 1).

"To continue to fail to do so would be a gross abdication of your responsibility to serve the people of Hong Kong and ensure their well-being and safety."

The 12 should be returned to Hong Kong immediately, be allowed to nominate legal representatives and given access to their families, they said in the letter.

The parliamentarians are from Australia, Canada, Denmark, France, Germany, Ireland, Italy, Japan, Lithuania, Myanmar, the Netherlands, New Zealand, Nigeria, Norway, Sweden, Uganda, the United Kingdom and the United States, as well as the European Union.

Mrs Lam has said that the 12 will have to face justice in the mainland and that her government will provide them with "needed and feasible" assistance.

Human rights groups and democracy activists have expressed fear over the conditions and treatment of the 12, with families and lawyers for them denied access.

Chinese authorities have insisted that the group be represented by officially appointed lawyers.