"The real purpose of the US is to destabilise and even bring Hong Kong to ruin. China has taken countermeasures against the US actions. The US should immediately stop interfering in Hong Kong affairs, otherwise it will only shoot itself in the foot," Sun told PTI in an interview.
His strong criticism of the US came at a time when China and the US were inching closer to seal a trade deal to end months of tariff war between the two economic powers.
"The spiral of violence in Hong Kong has plunged the territory into a technical economic recession and led to a surge of unemployment in consumer sector and tourism, and undermined the confidence of foreign investors in Hong Kong," said the Chinese envoy.
Since June, Hong Kong, a former British colony which was returned to China in 1997, witnessed massive anti-government protests after the China-backed administration of the city unveiled plans to allow extradition of people to mainland China.
Following the protests, the proposed legislation was scrapped in September, but demonstrations continued to rock Hong Kong as agitators demanded democracy for the autonomous region.
Hong Kong has some autonomy and China has been administering it under "one country, two systems" framework over the last 22 years.
The Chinese envoy also strongly criticised the US for bringing in the human rights and democracy law which provides for periodic review of autonomy in Hong Kong and links it with Washington's trade with the region.
"This is a blatant violation of international law and basic norms governing international relations, and a bold-faced interference in China's internal affairs, under the pretext of 'democracy' and 'human rights'," Sun said.
Asserting that Hong Kong is a part of China, he said the protests were an internal affair of China and that no one should underestimate the firm determination of the Chinese government to oppose "external interference" in the autonomous region.
Reflecting China's hardline position on the issue, he said no country should "misjudge" Beijing's resolve in implementing the principle of "one country, two systems" and safeguarding the country's "sovereignty, security and development interests".
"There is evidence showing that the interference, intervention and instigation of the external forces are behind the escalation of Hong Kong's situation today," he claimed.
"It has been exposed that diplomats from the US Consulate General in Hong Kong met with the heads of the so-called 'Hong Kong independence group'," he added.
The US has been severely critical of China on its handling of the protests in Hong Kong.
On whether the Trump administration's action following protests in Hong Kong will impact the trade deal being negotiated by the both sides, Sun said: "China's position on China-US economic and trade issue is consistent and clear."
"The trade frictions provoked by the US will only harm others without benefiting itself, and eventually harm the interests of the American people," he said.
"We hope the US side can meet the Chinese side halfway and reach a mutual beneficial and win-win agreement on the basis of mutual respect, equality and mutual benefit. This is in the interest of both countries, and the common expectation of the international community," he added.
On India's position on Hong Kong, the envoy suggested that New Delhi was adopting a fair and objective approach on the issue.
"China and India once jointly advocated the five principles of peaceful coexistence and opposed interference in other countries' internal affairs," Sun said.
"India has a large number of diaspora and enterprises in Hong Kong. It is not only in China's interest, but also in the common interest of all countries, including India, to maintain Hong Kong's prosperity and stability and uphold 'one country, two systems' policy," he said.
"I believe that is what our Indian friends with fair and objective attitude would like to see," he added.
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