BEIJING — China on Monday reiterated that COVID-19 origin-tracing issue is a scientific question and refuted the US "lab leak" accusation, calling it "purely blackmail and threat." US National Security Adviser Jake Sullivan warned on Sunday that China would face international isolation if it "fails" to conduct a genuine investigation" on its territories to pinpoint the source of the virus.The spokesman of the Chinese foreign ministry called on the US to stop "politicizing the issue of the coronavirus to tarnish China"s image." China has taken an open and transparent approach with respect to investigating the source of the virus, he said, adding that Beijing has been coordinating closely with the World Health Organization (WHO) in this regard. The US has repeatedly said that the virus leaked from a laboratory from Wuhan, a province in central China, where the killer germ had hit it hard with heavy casualties before the Chinese made a game-changer with a series of strict precautions that contained the germ outbreak. — Agencies