An office at the American consulate general in Guangzhou has been closed indefinitely after an employee found some suspicious powder in an envelope.
"On Monday, we found an unidentified substance in an envelope and we are now carrying out an investigation," said a consulate employee who wished to remain anonymous. "The office which was affected is closed, but not all the consulate offices are closed," he added.
The envelope was found at the consular section at the Tianyu Garden Building in Tianhe district, which handles visa applications and provides assistance to US citizens in southern China, the employee revealed.
The unidentified powder was discovered "in an area not accessible to the public" and precautionary measures were taken, he said.
"The front-desk service is closed, but some teams still operate without going to the office," the employee said.
The consular section was closed on Tuesday and Wednesday, the consulate said on its website  on Tuesday.
A two-day closure of the consular section could on average affect some 1,800 visa applications. The consulate general handles 121,000 visa applications per year, according to its website. 
Consular services "will resume as soon as we have completed our investigation in accordance with all our established procedures to handle this type of situation," Nolan Barkhouse, press spokesman at the US embassy in Beijing, said.
"We have notified Chinese authorities and we maintain close contact in case any assistance becomes necessary," Barkhouse said, adding that no one had been injured.
It is understood that the powder is not being tested in the mainland and that the person who was in contact with the powder did not show any signs of poisoning.
Suspicious white powder had previously been discovered at the Guangzhou consulate general, one person familiar with the situation said.
The now closed facility has 30 interviewing windows for 23 American staff and 100 locally-employed staff.
The consulate serves the provinces Guangdong, Hainan and Fujian and the Guangxi Autonomous Region.
Mimi Lau contributed reporting.