The different political opinions he is hearing from the lawmakers is useful for him to understand the political fabric of the city, says the Ohio-born diplomat who has focused on trade and economic affairs in his career.

He continues to draw the line carefully: “Stating an opinion about the United States’ aspirations for Hong Kong is not interfering in Hong Kong’s domestic political affairs. If I was out campaigning for specific candidates or issues that don’t have anything to do with the US, that will be interference. We don’t do that.”

But he also adds: “When mistakes are made or unwise policies take place, for the international community to comment on those is a natural thing.”

Among the “mistakes” he cites is Beijing’s handling of Hong Kong over the missing booksellers’ incident. While Tong has been vocal on the controversy, he is convinced that “one country, two systems” should remain the governing formula for the city.

Tong has come to the city at a time when Beijing is flexing its muscles over Hong Kong, alarmed at the support for pro- independence and localist advocates in the Legislative Council polls last year.

China’s worries about so-called “collusion with external forces” date back to the 1980s when China was negotiating with Britain over Hong Kong’s future. Late patriarch Deng Xiaoping cautioned that the city must be prevented from becoming a subversive base with people acting “under the veil of ­democracy”.

“Maybe there is not any proof,” Tong says.

The National Endowment for Democracy, a non-government organisation that Beijing loyalists have often accused of “meddling” with Hong Kong pan-democrats, is “mostly an independent organisation with some funding from Congress” to help promote ­democracy inside China – “a difficult task”, Tong notes.

Nathan Law Kwun-chung, a former Occupy student leader and now a lawmaker, says he has had a “very casual talk” with the envoy about his views on Hong Kong politics in general. “The consul was only there to gain more understanding,” he says.

Law, often a target for the leftist press which slams him for “colluding” with foreign forces when he testifies or speaks about Hong Kong to foreign parliamentarians, says he regards his activities as normal exchanges.

“The Chinese government wants to monopolise the narrative about Hong Kong. I’m only there to say what is right for the sake of the democratic ­movement,” he says.

Professor Lau Siu-kai, vice-chairman of the Chinese Association of Hong Kong and Macau Studies, a semi-official think tank, says while diplomats appear to act with caution, “their public statements and occasional meetings with pan-democrats often give the impression that they side with the opposition”.

What irks Beijing even more is individual politicians and semi-government organisations, especially those which do not have to report to Congress on their finances, Lau adds. “But because those semi-officials are not under their control, Beijing’s doubts about foreign interference can never be erased.”