More US citizens have been evacuated from China amid concerns that American government personnel and their families may be the target of “sonic attacks” by a rival country.

US state department officials said on Wednesday it had sent “a number of individuals” from its consulate in Guangzhou back to the US for “further evaluation and a comprehensive assessment of their symptoms”. Last month, a consulate worker in Guangzhou was found to have suffered a mild traumatic brain injury after reporting “abnormal sensations of sound and pressure” from late 2017 through to April 2018.

The department sent a team to Guangzhou in late May to examine other US staff and their families, and investigate possible links between their symptoms and those of US diplomats in Cuba last year, an incident that prompted Washington to pull its staff from the country and expel Cuban diplomats from the US.

The US secretary of state, Mike Pompeo, on Tuesday announced the formation of a task force to investigate unexplained health incidents among US government personnel and their families overseas.

The latest evacuation suggests what was previously described as an isolated case may turn into a wider diplomatic crisis, at a time when US-China ties are already at a low.

China’s foreign ministry on Thursday said it had not been informed by the US about the new cases. “If the United States communicates with us, we will adopt a responsible attitude to investigate this,” foreign ministry spokesperson Hua Chunying said at a regular news conference.

Hua said the government had investigated the incident reported in May and found no suspicious activity. “We haven’t found the cause or clues that would lead to the situation mentioned by the United States,” she said.

China’s foreign minister, Wang Yi, previously described the incident in May involving the US consulate worker as an “individual case”, and one Beijing hoped would not be “magnified, complicated or even politicised”.

When the US issued a health alert in May to US citizens in China to report any “unusual acute auditory or sensory phenomena”, it said it was not aware of other similar cases within or outside the US diplomatic community in China.

That has been disputed by Mark Lenzi, a security engineering officer at the consulate in Guangzhou, who, according to the New York Times, was among the personnel evacuated on Wednesday.

Lenzi, who lived in the same complex as the consulate worker who suffered brain trauma, said he had been hearing sounds like “marbles bouncing and hitting a floor” since April last year. That was followed by excruciating headaches and sleeplessness, symptoms his family also experienced. When he brought his concerns to his superiors, he was prescribed painkillers.

Lenzi sent an email to staff of the consulate criticising the fact that the first employee was evacuated in April, but US citizens weren’t alerted until a month later. The health alert suggested it was a single case. “They knew full well it wasn’t,” he told the paper.

When asked about Lenzi’s allegations, a US state department spokesperson said the government does not comment on specific cases out of privacy concerns.

“The state department has been and will continue to be diligent and transparent in its response to our employees’ concerns,” departmental spokesperson Heather Nauert said.