Blinken said the entire federal government is working to get to the bottom of the illness, which is said to have afflicted about 200 US diplomats, officials and family members overseas.

"To date, we don't know exactly what's happened and we don't know exactly who is responsible," Blinken said in an interview with MSNBC.

The Wall Street Journal on Thursday reported additional illnesses among officials serving in US diplomatic missions in Paris and Geneva, where the United States and Russia held security talks on Monday over a Russian troop build-up near the Ukraine border.

Blinken said the United States has raised the illnesses with the Russian side but still cannot make a determination about who was responsible.

The mysterious ailment carries symptoms including migraines, nausea, memory lapses and dizziness.

It was first reported among US officials in the Cuban capital in 2016.

Blinken said he has met with State Department employees around the world who described the illnesses and how these incidents disrupted their lives.

"There is no doubt in my mind that people have been directly and powerfully affected," he said.

"We are working overtime across the entire government to get to the bottom of what happened, who's responsible. And in the meantime to make sure that we're caring for anyone who's been affected and to protect all of our people to the best of our ability."

Australian Associated Press