"We are concerned that consular officers will not be allowed to attend Ms. Yu's trial in August 2014 despite the fact that under the 1982 bilateral consular convention between our two countries consular officials are permitted to attend such trials," U.S. Embassy spokesman Nolan Barkhouse said on Friday.
The relevant officials at China's foreign ministry did not immediately respond to a request for comment.
The U.S. embassy's comment draws more attention to the trial of Humphrey and Yu, which will held behind closed doors with consular officials and family barred, two family friends with knowledge of the matter told Reuters on Thursday.
ChinaWhys, the risk consultancy run by the couple, was employed by GSK in April 2013 to investigate an ex-employee suspected of sending anonymous emails, including the circulation of an intimate video of former GSK China head Mark Reilly with his girlfriend, as well as emails containing allegations of widespread bribery at the British drugmaker.
Three months later, authorities detained Humphrey and Yu for illegally buying and selling private information. Chinese authorities have not openly made a link between GSK and the case against ChinaWhys.