Hong Kong’s justice secretary has accused US President Donald Trump of infringing the principle of non-intervention under international law, arguing it is “completely false and wrong” to say the city has lost its autonomy.

Teresa Cheng Yeuk-wah also said that the national security law proposed for the city, which triggered the US’ controversial move, might not be retrospective in principle.

Cheng hit back hours after Trump revealed his government would begin eliminating special policy exemptions it granted to Hong Kong, following its earlier statement that the city was “no longer autonomous” from mainland China.

Trump’s announcement came a week after Beijing declared it planned a new security law tailor-made for Hong Kong prohibiting acts of subversion, secession, terrorism or conspiring with foreign influences in the city – a move that critics feared would effectively criminalise all forms of dissent and opposition activity.

Accusing Beijing of replacing its “promised formula of ‘one country, two systems’ with ‘one country, one system’” with the new legislative action, Trump said early on Saturday the US would take action to revoke Hong Kong’s preferential status as a separate customs and travel region and sanction Chinese and Hong Kong officials “directly or indirectly involved in eroding Hong Kong’s autonomy”.

Cheng said on Saturday the basis of the US action was “completely false and wrong”.

“People often forget that Hong Kong is part of China. We are one country, and without one country, there isn’t any basis to talk about two systems,” Cheng told the media after appearing on a radio programme.

“In so far as national security is concerned, as in any other country in the world, this is a matter that belongs to the central authorities … the US passed their national security laws, so can China.”

She said any attempt, by coercion or whatever means, to interfere with the sovereign right of a state to pass their own national security law was “arguably infringing on the principle of non-intervention under public international law, and that is not acceptable”.

Asked whether she was concerned about Trump’s pledge to sanction officials on the mainland and in the city, Cheng said it was not only legal but necessary for the central authorities to take action, since Hong Kong had not been able to pass its own national security legislation as required under Article 23 of the Basic Law.

Concerning the details of the law proposed by Beijing, to be further drafted and passed by the National People’s Congress Standing Committee (NPCSC) by August, Cheng said it should in principle be non-retrospective.

“In general, criminal laws have no retrospective power as [they are] regulated by human rights law and international common practice … but of course, there is an exception to every principle. I can only talk about the principle,” she said.

She also called on the public not to be “overly worried” as it was written down as a principle of the proposed law that the city’s prosperity and its residents’ freedoms would be safeguarded.

But Elsie Leung, a former member of the Basic Law Committee, said whether the law was retrospective would depend on social circumstances, as to avoid some may endanger national security before the law took effect.

“If many have resorted to acts endangering national security during the ‘window period’, will there be a need to make it retrospective? If there are no such acts, the law in general should not be retrospective,” Leung said on a TV programme.

Separately, security minister John Lee Ka-chiu said it was too early to discuss how the law would be enforced and what would be the role of mainland’s agencies, as the clauses of the legislation had not yet been drafted.

But he said organisations enforcing national security in Hong Kong must follow local laws and “not overstep their boundaries”.

He added that local law enforcement agencies would need “appropriate training” because they lacked experience in dealing with national security matters.

“The money the US earns from Hong Kong is a trade surplus every year, at least US$30 billion,” Lee said on a radio programme on Saturday.

He said all sides would be affected should the US go ahead with scrapping its preferential trade treatment for Hong Kong, but added American businesses would be hurt most.

Lee added that the US was targeting China as it saw Beijing was challenging its status as the sole superpower.

“They will not succeed in threatening government officials with these means, because [implementing the national security law] is a black-and-white matter,” he said.

Lee argued that Hong Kong’s right to be treated as a separate tax region from the mainland was stipulated in the Basic Law – often dubbed the city’s mini-constitution – and was recognised by the World Trade Organisation.

“So, it is not unilateral,” he said.