The United States is warning its citizens against travelling to Cuba, and is ordering more than half of its personnel to leave the island in response to what the US describes as "specific attacks" on diplomats.

The dramatic response is a significant blow to the diplomatic relationship between the two countries, which is notably delicate. The new travel warning will reportedly be issued Friday, and will say that some of the attacks occurred in Cuban hotels, although no tourists are known to have been targeted.

The US embassy will lose roughly 60 per cent of its American staff, and will stop processing visas there indefinitely, according to American officials.

The travel warning could have a significant impact on the Cuban economy, which relies heavily on tourism and has seen growth in recent years as a result of relaxed US restrictions on travelling to the island.

The announcement comes roughly a year after American diplomats in Cuba began describing unexplained health problems. US investigators still haven't been able to determine who is behind those attacks, which impacted at least 21 diplomats and their families. Some of the injuries were as serious as traumatic brain injury and permanent hearing loss. The State Department has, up until now, generally avoided calling those incidents "attacks".

The decision to draw down the embassy was made overnight by Secretary of State Rex Tillerson, who was travelling to Cuba. Mr Tillerson had reportedly considered other options, including a full embassy shutdown, but opted for the less severe reaction. Donald Trump reviewed all of those options with Mr Tillerson earlier in the week.

The US notified Cuba of its decision through its embassy in Washington. The Cuban embassy later said that the reduction in staffing was "hasty", and that the move would hurt diplomatic ties.

Pictures of everyday life in Cuba Show all 20 1 /20 Pictures of everyday life in Cuba Pictures of everyday life in Cuba A man rides his modified bicycle past a vintage American car in Havana Getty Images Pictures of everyday life in Cuba A taxi sits parked by Ancon Beach waiting for returning bathers in Trinidad Getty Images Pictures of everyday life in Cuba Afrocuban carnival group "Los componedores de batea" performing in the streets of La Habana Vieja Rex Pictures of everyday life in Cuba Pastel colours for an ice-cream place and a vintage American car in Cienfuegos after sunset Rex Pictures of everyday life in Cuba A man on the phone in a bookshop in Old Havana (Habana Vieja) selling books and displaying propaganda poster of the Cuban Revolution Rex Pictures of everyday life in Cuba Street Musicians in Santiago De Cuba Rex Pictures of everyday life in Cuba A man works to repair his classic American car after it broke down along the Prado, a wide avenue that runs from Parque Central to the Malecon seafront highway, in Havana Getty Images Pictures of everyday life in Cuba Members of the 'Ladies in White,' a group founded by the partners and relatives of jailed dissidents that regularly protests against the Cuban government, demonstrate on the streets of Havana Getty Images Pictures of everyday life in Cuba Street vegetables vendor in Havana Getty Images Pictures of everyday life in Cuba The sun setting through the palm trees and creates long shadows on the pool deck at this resort in Cuba Varadero Rex Pictures of everyday life in Cuba General view of a street in Havana Getty Images Pictures of everyday life in Cuba A girls plays on a street in Havana Getty Images Pictures of everyday life in Cuba Girls walk past graffiti art along the Paseo de Marti, the wide boulevard that runs through the heart of the historic Old Havana neighborhood in Havana Getty Images Pictures of everyday life in Cuba A woman smokes her Havana cigar Getty Images Pictures of everyday life in Cuba A man harvests tobacco leaves for drying at a tobacco drying house on a co-op plantation in Pinar del Rio Getty Images Pictures of everyday life in Cuba Men play chess on a street in Havana Getty Images Pictures of everyday life in Cuba Locals take part in a gay parade in Havana Getty Images Pictures of everyday life in Cuba Scene of the Memories Paraiso Azul resort in Santa Maria Key Getty Images Pictures of everyday life in Cuba Beach on the Bay of Pigs, Zapata Peninsula Pictures of everyday life in Cuba Divers swimming above coral reef in Caribbean Sea Rex

Although the Trump administration has refrained from blaming the Cuban government for the attacks, the decision to pull diplomats is a significant setback to the thawing relations between the two countries. The two had no formal diplomatic relations for a half century, even though they are just 90 miles apart from one another. That began to change in 2015, when then-President Barack Obama and Cuban President Raul Castro announced a restoration of ties, and the opening of embassies. That restoration of ties included an easing of travel and commerce restrictions. Mr Trump has reversed some of those changes, but has left most of the new policies intact.

The attacks on diplomats have been incredibly difficult for investigators to understand. Because symptoms vary widely between person, a single method of attack has been difficult to pinpoint. Investigators initially suspected some sort of futuristic "sonic attack", but searches of the homes and hotels where diplomats were targeted turned up no devices. Some diplomats described hearing various loud noises or feeling vibrations when the incidents occurred. Still others did not notice anything out of the ordinary when they were attacked.