Heather Nauer, a spokeswoman for the US State Department, said "some US government personnel" working at the US embassy in Havana, Cuba on official duty had been experiencing "physical symptoms".
An FBI official told CBS News the bureau has been looking into the issue.
The American authorities have expelled two Cuban diplomats following a mysterious "attack" on staff in the United States embassy in Havana. At the moment, the United States can not blame any individual or country in the incident, she added. "The Americans eventually came home". She didn't give the number of the Americans affected.
As a result, the U.S. asked two Cuban officials on 23 May in Washington to leave the country and they have done so, Ms Nauert said, an action that Cuba described as "unjustified".
"We requested their departure as a reciprocal measure since some United States personnel's assignments in Havana had to be curtailed due to these incidents", she said.
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Relations with the country have become a significant issue in U.S. politics, he explained, thanks in part to the population of Cuban exiles in Florida, a battleground state in USA elections.
The incident caused a diplomatic row between the two countries, leading to two Cuban diplomats being expelled from Washington in May.
Nauert further emphasized Wednesday that "the Cuban government has a responsibility and obligation to protect our diplomats under the Geneva Convention". When asked specifically what the incidents were, Nauert replied, "That's what we're calling it. There is a lot we still don't know".
As to why it has taken officials so long to do something about this, Nauert said that "it took time to figure out what it was, and this is still ongoing".
"We first heard of these incidents back in late 2016", Nauert said, adding that there were no "definitive answers about the source or cause".
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For the players it must be the same. "We are now trying to work in different directions". Everything is a little more hard .
It comes a day after the U.S. said its diplomats in Havana were experiencing odd physical symptoms.
The U.S. government has said personnel at the U.S. embassy in Havana were evacuated back home for hearing problems and other symptoms over the past six months. On May 23rd, two Cuban diplomats were asked to leave the United States in an apparent response to American diplomats being forced to come home for medical reasons.
Cuban officials have pledged to investigate the incidents as well and take appropriate action, the officials told the Post.
The Cuban Foreign Ministry statement also called the USA decision to expel two Cuban diplomats based in Washington "unjustified and baseless".
The Cuban Ministry of Foreign Affairs on Wednesday denied any involvement in the attack, saying the decision to remove the Cuban diplomats was "unjustified and unfounded".
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