The Hong Kong government has threatened to "take action" against a new independence party and China slammed the group, which said Thursday it had been blocked from registering for political reasons.

The Hong Kong National Party, made up of 30-50 students and young professionals, launched Monday saying it is tapping in to the semi-autonomous city's increasing desire to break away from the mainland.

There have been growing calls for independence from Hong Kong's youth over fears Beijing is seeking to curb freedoms in the city.

The Hong Kong government said advocating independence was against the city's mini-constitution and would "undermine the stability and prosperity of Hong Kong".

"The SAR government will take action according to the law," it said in a statement late Wednesday. Officials told AFP they had "no further comment" on what that might entail.

The Hong Kong National Party hit back Thursday saying: "The...allegation that we may incur criminal liability for advocating the independence of Hong Kong is absurd."

It added that if anyone was charged for simply speaking about independence there would "be a fundamental constitutional and political crisis".

China's State Council Hong Kong and Macau Affairs Office voiced "resolute opposition" to calls for independence, according to official news agency Xinhua.

It said the Hong Kong government had "refused to register" the new party, a move it termed "proper".

A member of the pro-independence party confirmed to AFP Thursday the group had been blocked from registering as a company, which could make it difficult for it to stand in any election.

Kris Lai said the party had approached the Companies Registry via an accounting firm, which told them "that because of political problems they turned us down".

- 'Unconstitutional and idiotic' -

Hong Kong solicitor and legislator James To said a party had to be officially registered as a company or society in order to feature on a ballot paper.

"It will be difficult" for candidates to run without the registration, To said.

However, some small parties told AFP they had succeeded in doing so, apparently flouting the rules.

The city's electoral office could give no immediate comment on whether the failure to register as a company would prevent the Hong Kong National Party from standing.

Hong Kong's freedoms are protected by a 50-year agreement signed when Britain handed the city back to China in 1997, but there are fears those freedoms are dying.

Concerns have been fuelled by the disappearance of five Hong Kong booksellers, known for salacious titles critical of Beijing, who later turned up on the mainland.

Four of the men are now under criminal investigation in China and the fifth is flitting between Hong Kong and the mainland, where he says he is "assisting" with the investigation.

The failure of mass protests in 2014 to win political concessions from Beijing have also led to the rise of "localism", dominated by young activists who want more autonomy.

In a strongly worded editorial Thursday, the government-published China Daily accused the new party of trying to advocate separation through "illegal means".

"Do they know what they advocate is illegal, unconstitutional and idiotic?" it said.