On Thursday, China responded to a Vice News report which claimed that US diplomats in the country were complaining after having been administered the swab tests.
"The State Department never agreed to this kind of testing and protested directly to [China's Ministry of Foreign Affairs] when we learned that some staff were subject to it," a state department spokesperson told CNN in a statement on Thursday.
"We have received assurances from [the ministry] that this was in error and that diplomatic personnel are exempt from this testing requirement. We have instructed staff to decline this test if it is asked of them (as was done in the past)," the statement added.
China's Ministry of Foreign Affairs said it was unaware of anal swab tests being administered.
"As far as I know, and I have also checked with my colleagues, China has never asked US diplomats in China to take anal swab tests," ministry spokesperson Zhao Lijian told reporters at a daily briefing on Thursday.
The US diplomatic source told CNN that US representatives in China have been "negotiating with the Chinese for a while" to avoid their staff being subjected to such tests.
Chinese state media have been reporting in the last few weeks that anal swab tests were being used in some cases. Some doctors and studies in China consider them more reliable than nasal or throat swab tests.
"Some asymptomatic patients or those with minor symptoms recover fast [from Covid-19], and it's possible that throat tests won't be effective for these people in three to five days," Li Tongzeng, an infectious disease doctor in China, told CNN.
"Researchers have shown that for some infected people, the duration time of positive nucleic results lasted longer on their excrement and anal swab test [samples] than those on the upper respiratory tract. Therefore, adding anal swab tests can improve the positive detection rate of the infected," he said.
It is unclear whether anal swabs are more efficient at detecting Covid-19.