National security suspects could be sent to the mainland for their trials if the United States slaps sanctions on Hong Kong judges and prosecutors, a former delegate to the country's top legislature warned on Tuesday.

Tam Yiu-chung, a former member of the National People's Congress Standing Committee, said Hong Kong would be left with no option but to take such a step if the US Congress passes a bill to sanction another 49 SAR officials tasked with safeguarding national security.

Tam said the bill has bipartisan support, so there's a need to warn Washington "not to mess around".

He added that the American lawmakers might think that they are helping national security suspects in Hong Kong with their action, but they are actually hurting them.

"My own observation is that, we'd be forced to [send national security suspects to the mainland]. We don't have much of a choice," he said on a Commercial Radio programme.

"[If] the Americans pass the bill and our judges and prosecutors are sanctioned, you tell me: would they be under pressure? Of course they'd be under pressure."

Tam said Article 55 of the national security law was designed as a safety mechanism to allow suspects to be tried across the border if necessary.

Meanwhile, all SAR lawmakers issued a joint statement strongly condemning "brutal interference in Hong Kong affairs and blatant interference with the judicial system" by the US politicians.

The SAR legislators said they will support the government in enforcing the security law and safeguarding national sovereignty and security together with the Hong Kong people.

"Currently I do not anticipate any specific meetings with US government officials," Financial Secretary Paul Chan, who will attend the upcoming Apec summit in San Francisco on behalf of Chief Executive John Lee, told reporters on Tuesday when asked about the possible sanctions.

"As to the sanctions that you mentioned, of course this kind of threatening of unilateral sanctions is totally unacceptable. We will never compromise and we never fear this threat."