Three diplomats became ill in Swiss and one in the French capital last summer, with around 200 people affected by more than five years.
State Secretary Antony Blinken said the American government worked to get to the basis of mystery.
There are worries, the enemy may have a targeted diplomat with a microwave.
Mr. Blinken said the problem was raised with Russia but there was no determination made.
Havana was the first Emergedin Cubain 2016 and has affected diplomats, spies, officials and US family members, as well as a number of Canadian diplomatic staff deployed in Cuba.
Some people who experience syndrome are left with dizziness and fatigue for months.
The latest cases include one person evacuated from Switzerland to the US for maintenance, media reports said.
Mr. Blinken told MSNBC in an interview that he had met employees of the Ministry of Foreign Affairs throughout the world who had described their disease and how their lives were disturbed.
People, he said, have “affected directly and strongly”.
Until now, we do not know exactly what happened and we did not know exactly who was responsible,” he told the announcer.
We work overtime in all governments to get to the basis of what happens, who is responsible,” he said.
Russia, China and the US itself are all said to have examined the microwave for military purposes but Moscow has rejected the accusation using a “microwave weapon” against the US mission as a “fantastic hypothesis”.
A more innocent theory, but also has not been proven that those who are sick suffer from bulk conditions caused by some of the underlying situations.