HONG KONG - Authorities on Tuesday told a political party that advocates independence for Hong Kong the group might be banned on national security grounds, in one of the most severe steps against separatist voices since the territory returned to Chinese rule in 1997.

In a letter to the leader of the Hong Kong National Party, 27-year-old Andy Chan, the territory's security bureau said the group has three weeks to make a case for why it should be allowed to operate.

An official believes the party should be dissolved “in the interests of national security or public safety, public order or the protection of the rights and freedoms of others,” the letter said.

It cited a national security law that has not been invoked since 1997.

The bespectacled Chan, who wore a crew-neck T-shirt and black wristwatch as he fielded phone calls on Tuesday, told The Associated Press that police approached him with documents detailing his speeches and activities since the party's formation in 2016.

The party was founded in response to frustration about Beijing's treatment of Hong Kong. Despite a promise of autonomy, activists complain mainland influence over its democratic elections is increasing.

Hong Kong National Party has always been an enemy of China,” Chan said. He said the group has never registered with police because they do not want to disclose all their information.

Chan said he suspected the proposed ban is related to a trip he made to Taiwan earlier this month, when he held a news conference with politicians there to talk about the human rights situation in Hong Kong.

Hong Kong's security secretary, John Lee, said police recommended banning the group.

Hong Kong has freedom of association, but that right is not without restriction,” Lee said at a news conference. He said national security “means safeguarding the territorial integrity and independence of the People's Republic of China.”

Chan and other pro-independence candidates were disqualified from 2016 elections to the Hong Kong legislature after they refused to sign a pledge saying Hong Kong is an inalienable part of China. The Hong Kong National Party has never held any seats on the council.

A statement on the party's Facebook page called Tuesday's letter an effort to “pelt pebbles at the Hong Kong people and independence movement.”

“For what is their 'national security,' their 'laws,' and their banning of operations but a purely political decision to silence those who dare to represent the true interests of the Hong Kong people and nation?” the party wrote.

Amnesty International condemned the proposed ban.

“The authorities must stop using vague laws to intimidate people who hold different political views,” the human rights group said.