TAIPEI, Taiwan (AP) — Beijing has accused two Chinese citizens of spying for Britain, in the latest test of a relationship that has grown increasingly fraught.

China and the U.K. have clashed over Beijing’s clampdown on free speech and open elections in Hong Kong, a former British territory that was guaranteed its own economic and political freedoms for 50 years after its handover to Chinese rule in 1997.

In a message on social media on Monday, the Ministry of State Security, China’s main intelligence service, said it uncovered a major espionage case involving a couple identified only by their surnames, Wang and Zhou, who were allegedly recruited by Britain’s foreign intelligence agency, M16. It said Wang had gone to Britain as a student in 2015 and was later joined by his wife. It said Wang was given hotel rooms, trips around the country and financial incentives.

It said the couple worked for the Chinese government in a “central state agency” and handled government secrets, which they passed to MI6. No information was given about what specific information the couple may have provided. The ministry said the case was still under investigation and gave no word on the location of the couple.

There was no immediate comment from Britain.

Last month, Britain said two men would go on trial on suspicion of collecting sensitive information for Hong Kong authorities. A third suspect, 37-year-old Briton Matthew Trickett, was also charged in the case, but was found dead in a park under what police said were unexplained circumstances.