Two members of a World Health Organization-led (WHO) team that travelled to China's Wuhan city to investigate the origins of COVID-19 remained behind in Singapore after testing positive for coronavirus antibodies.
The team of 15 had all tested negative for the disease prior to leaving their home countries, and underwent further testing while in transit in Singapore.
The results of nucleic acid tests were negative but showed two of the members had coronavirus antibodies, the Geneva-based agency said in a tweet.
"They are being retested for both IgM and IgG antibodies," the WHO said.
It is the latest setback for a mission beset by delay as well as concern over how much access the team will get.
The rest of the team arrived in Wuhan from Singapore late on Thursday morning on a budget airline and they were expected to head into two weeks of quarantine.
"Relevant epidemic prevention and control requirements and regulations will be strictly enforced," Chinese foreign ministry spokesman Zhao Lijian told a briefing in response to a question about the two team members.
The team tasked with investigating the origins of the novel coronavirus that sparked the global pandemic had originally been scheduled to arrive earlier this month.
China's delay of their visit drew rare public criticism from the head of the WHO.
The group left the airport terminal in Wuhan through a plastic quarantine tunnel marked "epidemic prevention passage" for international arrivals and boarded a cordoned-off bus that was guarded by half a dozen security staff in full protective gear.
The coronavirus was initially linked to a seafood market in the central city of Wuhan.
Team members did not speak to reporters, although some waved and took pictures of the media from the bus as it departed.
The United States, which has accused China of hiding the extent of its initial outbreak a year ago, has called for a "transparent" WHO-led investigation and criticised the terms of the visit, under which Chinese experts have done the first phase of research.
The team arrived in China as the country battles a resurgence of coronavirus cases in its north-east after managing to nearly stamp out domestic infections in recent months.