The State Department said it was taking the reports seriously, but did not yet know the cause, and warned staff to move to a safe place if they encountered any “unusual acute auditory or sensory phenomena accompanied by unusual sounds or piercing noises”.
One US official was diagnosed with mild brain trauma, the same injury that affected the Cuban embassy staff.
The State Department has warned that US diplomats should alert their mission’s medical staff “if they note new onset of symptoms that may have begun in association with experiencing unidentified auditory sensations”.
It said it had sent a team to Guangzhou and set up a task force to oversee the response to the mystery attacks in China and Cuba.
Cuba has denied targeting embassy staff, and the US has not blamed the country’s government for the suspected attacks.
According to specialists, symptoms of a sonic attack may include dizziness, headaches, vomiting, bowel spasms, vertigo, permanent hearing loss and even brain damage.