The United States cut its diplomatic presence in Cuba today by more than half and warned US citizens not to visit because of mysterious "attacks" that have caused hearing loss, dizziness and fatigue in US embassy personnel.

The US embassy in Havana will halt regular visa operations for Cubans seeking to visit the United States and offer only emergency services to US citizens, steps that may further erode the US-Cuban rapprochement begun by former President Barack Obama.

The partial evacuation, while depicted as a safety measure, sends a message of US displeasure over Cuba’s handling of the matter and delivers another blow to Mr Obama's policies of engagement with Cold War foe Cuba.

Cuba's Foreign Ministry chief for US Affairs Josefina Vidal said: "We consider the decision announced today by the US government through the State Department is hasty and will affect bilateral relations."

In a briefing on state-run television, Ms Vidal added Cuba was still keen to continue active cooperation with US authorities to clarify what happened.

Officials in US President Donald Trump's administration stressed the United States was maintaining diplomatic ties with Cuba.

Twenty-one US embassy employees in Cuba have been injured and reported symptoms such as hearing loss, dizziness, headache, fatigue, cognitive issues, and difficulty sleeping, the US State Department said.

"Until the government of Cuba can ensure the safety of our diplomats in Cuba, our embassy will be reduced to emergency personnel in order to minimise the number of diplomats at risk of exposure to harm," Secretary of State Rex Tillerson said in a statement.

The Cuban government has denied any role and is investigating.

A senior US State Department official told reporters neither the US nor Cuban governments had been able to identify who was responsible but stressed that "the government of Cuba is responsible for taking all appropriate steps to prevent attacks on our diplomatic personnel in Cuba".

In a travel warning, the US State Department bluntly said: "Because our personnel's safety is at risk, and we are unable to identify the source of the attacks, we believe US citizens may also be at risk and warn them not to travel to Cuba."

The US State Department said the attacks on US embassy personnel had occurred at "US diplomatic residences and hotels frequented by US citizens".

In June, Mr Trump vowed to partially roll back the detente with Cuba agreed by his Democratic predecessor, Mr Obama, and called the Cuban government "corrupt and destabilising" in his address to the United Nations General Assembly this month.

Bert Hoffman, a Latin American expert at the German Institute of Global and Area Studies in Hamburg, Germany, suggested Washington's decision may have been motivated by Mr Trump's desire to pursue a tougher policy with the Communist Party-run country.

"The incident seems just a pretext to toughen US-Cuba policy," Mr Hoffman said.

The analyst said the "de facto visa restrictions" resulting from the suspension of normal visa operations could call into question the US-Cuban migration accord hammered out in the 1990s under which the United States grants 20,000 visas per year in return for Cuba preventing illegal emigration.

Engage Cuba, a Washingon-based lobbying group, said the decision was "puzzling" given that American travelers had not been targeted.

It said halting the visa process in Cuba and discouraging Americans from going there "will divide families and harm Cuba's burgeoning private sector, civil society groups and efforts to improve human rights on the island".