The United States is still investigating what caused dozens of U.S. government employees in China and Cuba to become mysteriously ill, according US Secretary of State Mike Pompeo.

More than 40 US government employees were affected by the incidents,which took place between late 2016 into 2018 and came to be known as the "Havana syndrome".

Speaking at a news conference at the State Department, Mr Pompeo said "significant US government resources" had been dedicated to solving the mystery but there was still no complete analysis and only theories about how the incidents came to pass.

"We've done a lot of work to try and identify how this all took place and we continue to try and determine precisely the causation of this, while doing our best to make sure we're taking care of the health and safety of these people," Mr Pompeo said.

Dozens of staff in Cuba in 2017 reported symptoms that included hearing loss, ringing in their ears, vertigo, headaches and fatigue, a pattern consistent with mild traumatic brain injury and initially described as the result of "sonic" attacks or health attacks of some sort.

The State Department said in June 2018 that it had brought home diplomats from Guangzhou, China, over concern they were suffering similar symptoms.

The incidents prompted the administration of US President Donald Trump to say the diplomats were attacked by some sort of secret weapon.