WASHINGTON/HAVANA, Aug 9 (Reuters) - Cuba said on Wednesday it was investigating allegations by the United States that unspecified "incidents" caused physical symptoms in Americans serving at the U.S. Embassy in Havana, after two Washington-based Cuban diplomats were expelled.
"Cuba has never, nor would it ever, allow that the Cuban territory be used for any action against accredited diplomatic agents or their families," the foreign ministry said in a statement late on Wednesday. "It reiterates its willingness to cooperate in the clarification of this situation."
Havana said it had started a "comprehensive, priority and urgent investigation" into the alleged incidents after it had been informed of them by the embassy in February.
Earlier on Wednesday, U.S. State Department spokeswoman Heather Nauert told reporters that the exact nature of the incidents was unclear, but Americans serving in Cuba had returned to the United States for non life-threatening "medical reasons."
RELATED: Daily life in and around Cuba
Daily life in and around Cuba
Daily life in and around Cuba
A vintage car decorated with a Cuban flag and carrying tourists waits in line to fill up with fuel at a gas station along the seafront boulevard "El Malecon" in Havana, January 12, 2015. REUTERS/Alexandre Meneghini (CUBA - Tags: TRANSPORT BUSINESS COMMODITIES ENERGY SOCIETY TRAVEL)
Tourists from Colombia look at posters in a street arts fair in Havana, February 20, 2016. Picture taken February 20, 2016. REUTERS/Alexandre Meneghini
Yalis, a 5-year old dog, barks at his owner as he jumps into the sea at the seafront Malecon in Havana, March 16, 2016. REUTERS/Alexandre Meneghini
A taxi driver drives a vintage car in downtown Havana, January 16, 2015. The United States rolled out a sweeping set of measures on Thursday to significantly ease the half-century-old embargo against Cuba, opening up the country to expanded travel, trade and financial activities. REUTERS/Alexandre Meneghini (CUBA - Tags: POLITICS SOCIETY TRANSPORT)
Bici taxi driver Yosvani Gomes, 39, lifts the curtains of his vehicle after a rain in downtown Havana January 20, 2015. Cuba will tell the United States in face-to-face talks this week it wants to be removed from the U.S. list of state sponsors of terrorism before restoring diplomatic relations, a senior foreign ministry official said on Tuesday. REUTERS/Alexandre Meneghini (CUBA - Tags: POLITICS SOCIETY)
Yiliana Benitez, 33, works at the H. Upmann cigar factory in Havana, February 26, 2015. Cuban cigar-maker Habanos S.A. envisions gaining 25 percent to 30 percent of the U.S. premium cigar market if the United States lifts its trade embargo on Cuba, potentially selling 70 million to 90 million cigars per year, the company said on Monday. The prospect of the United States lifting is 53-year-old embargo improved after the United States and Cuba announced on December 17 their intention to restore diplomatic relations. REUTERS/Alexandre Meneghini (CUBA - Tags: BUSINESS COMMODITIES)
Cowboy Ariel Peralta (C), 25, watches a rodeo show at the International Livestock Fair in Havana March 22, 2015. Picture taken March 22. REUTERS/Alexandre Meneghini TPX IMAGES OF THE DAY
U.S. artists and art curators who came to visit the 12th Havana Biennial walk in downtown Havana, May 29, 2015. The United States formally dropped Cuba from a list of state sponsors of terrorism on Friday, an important step toward restoring diplomatic ties but one that will have limited effect on removing U.S. sanctions on the Communist-ruled island. REUTERS/Alexandre Meneghini
Cuba's Capitol, or El Capitolio as it is called by Cubans, is seen in Havana, July 1, 2015. The United States and Cuba on Wednesday formally agreed to restore diplomatic ties that had been severed for 54 years, fulfilling a pledge made six months ago by the former Cold War enemies. U.S. President Barack Obama and Cuban President Raul Castro exchanged letters agreeing to reopen embassies in each other's capitals, with the Cubans saying that could happen as soon as July 20. The Capitolio, which resembles the U.S. Capitol in Washington, was built in 1929 and was the seat of the government until after the 1959 revolution. REUTERS/Alexandre Meneghini
Tourists take pictures of a statue representing the Republic at Cuba's Capitol, or El Capitolio as it is called by Cubans, in Havana July 9, 2015. Cubans are once again touring their Capitol, an imposing structure previously shunned as a symbol of U.S. imperialism but now undergoing renovation and set to reopen as the new home of the Communist government's National Assembly. Picture taken July 9, 2015. REUTERS/Alexandre Meneghini
Dancer Cristian Perez, 20, (R) and informatics student Ariana Dexido, 17, dance near the sea in Havana, Cuba, July 12, 2015. Picture taken July 12, 2015. REUTERS/Alexandre Meneghini TPX IMAGES OF THE DAY
Mechanic and salsa dance instructor Ariel Domninguez, 26, (L), gives a class to Jarman Frash, 25, a medical student from Germany in Havana, February 4, 2015. REUTERS/Alexandre Meneghini TPX IMAGES OF THE DAY
Tourists dance during a salsa class at the beach in Varadero, Cuba, August 26, 2015. Cubans are flocking to the beach in record numbers before a possible end to the U.S. travel ban that would open the gates to American tourists and bump up prices. Picture taken on August 26, 2015. REUTERS/Alexandre Meneghini
Retiree Madeline Barcelo swims at the beach with her granddaughter in Varadero, Cuba, August 26, 2015. Cubans are flocking to the beach in record numbers before a possible end to the U.S. travel ban that would open the gates to American tourists and bump up prices. Picture taken August 26, 2015. REUTERS/Alexandre Meneghini TPX IMAGES OF THE DAY
Human resources worker Carmen Oivedo (R) talks to her daughters during their vacations at the beach in Varadero, Cuba, August 26, 2015. Cubans are flocking to the beach in record numbers before a possible end to the U.S. travel ban that would open the gates to American tourists and bump up prices. Picture taken on August 26, 2015. REUTERS/Alexandre Meneghini
Cuban tourists sail in a rented sailboat at the beach in Varadero, Cuba, August 26, 2015. Cubans are flocking to the beach in record numbers before a possible end to the U.S. travel ban that would open the gates to American tourists and bump up prices. Picture taken on August 26, 2015. REUTERS/Alexandre Meneghini
Pre-university students walk in downtown Havana to mark the first day of class for the 2015-2016 course, September 1, 2015. Universal free education is one of the pillars of the socialist society built in Cuba since Fidel Castro's 1959 revolution. REUTERS/Alexandre Meneghini
Cuban soldiers hold flags of Cuba and Youth Communist league during a ceremony in Havana November 27, 2014, marking the anniversary of the deaths of student leaders killed during the fight against Spanish colonial rule. REUTERS/Alexandre Meneghini (CUBA - Tags: ANNIVERSARY POLITICS MILITARY TPX IMAGES OF THE DAY)
A picture of former Cuban President Fidel Castro is seen inside a post office in Havana, December 11, 2015. Cuba and the U.S. have agreed to restore direct postal service after a half-century rupture in one of the first bilateral deals since the former Cold War foes re-established diplomatic ties in July. REUTERS/Alexandre Meneghini
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The United States first learned of the issues at the embassy in late 2016, she said.
"We don't have any definitive answers about the source or the cause of what we consider to be incidents," Nauert said. "It's caused a variety of physical symptoms in these American citizens who work for the U.S. government. We take those incidents very seriously, and there is an investigation currently under way."
As a result, the United States on May 23 asked two Cuban officials in Washington to leave the country and they have done so, Nauert said, an action that Cuba described as "unjustified."
"What this requires is providing medical examinations to these people," Nauert said. "Initially, when they'd started reporting what I will just call symptoms, it took time to figure out what it was, and this is still ongoing. So we're monitoring it."
A U.S. government official said several colleagues at the U.S. embassy in Havana were evacuated back to the United States for hearing problems and other symptoms over the past six months. Some subsequently got hearing aids, said the official, who spoke on condition of anonymity.
Washington and Havana re-established diplomatic relations in 2015 after more than five decades of hostilities, re-opening embassies in each other's capitals and establishing a new chapter of engagement between the former Cold War foes.
President Donald Trump rolled back part of his predecessor Barack Obama's policy toward Cuba, but has left in place many of the changes, including the re-opened U.S. Embassy in Havana. (Reporting by Mohammad Zargham and Yeganeh Torbati in Washington and Sarah Marsh in Havana; Editing by G Crosse, Toni Reinhold)