The first minors convicted under the national security law were sentenced Saturday to detention in a training centre by a judge who said their calls to overthrow China's government must be met with deterrence.

A 16-year-old girl and three 17-year-olds were members of a little-known pro-independence group that called itself "Returning Valiant" and promoted a violent uprising against China at street booths and on social media last year, the court heard.

They were charged with "conspiracy to incite subversion" under a sweeping national security law that Beijing imposed on the city to stamp out dissent after huge and sometimes violent democracy protests three years ago.

"We are alarmed by the sentencing on Saturday of another five people -- four of them minors -- under the National Security Law," UN Human Rights Office spokeswoman Ravina Shamdasani told reporters in Geneva.

The rights office and a number of UN human rights mechanisms have repeatedly expressed concerns over the negative impact of the law on fundamental rights and freedoms in Hong Kong.

The UN Human Rights Committee in July urged the authorities to take concrete steps to repeal the National Security Law and, in the meantime, refrain from applying it.

"We regret the continued application of the National Security Law, including against children, in spite of the clear recommendations of the Human Rights Committee," said Shamdasani.

"We urge the authorities to bring the Hong Kong SAR's legislation and practice fully into compliance with its international human rights obligations."

All five of those charged were sentenced to up to three years at a training centre, a rehabilitation-focused detention facility which can be a sentencing option for teens aged 14 to 20.

Hong Kong's once-popular democracy movement has been dismantled both by the security law and by prosecutors deploying a sedition law.

More than 210 people have been arrested under the law, with nearly 130 formally charged, mostly for political views and speech.