WASHINGTON (AP) — The United States is warning Americans against visiting Cuba and ordering more than half of U.S. personnel to leave the island, senior officials said Friday, in a dramatic response to what they described as “specific attacks” on diplomats.

The decision deals a blow to already delicate ties between the U.S. and Cuba, longtime enemies who only recently began putting their hostility behind them. The embassy in Havana will lose roughly 60 percent of its U.S. staff, and will stop processing visas in Cuba indefinitely, the American officials said.

In a new travel warning to be issued Friday, the U.S. will say some of the attacks have occurred in Cuban hotels, and that while American tourists aren’t known to have been hurt, they could be exposed if they travel to Cuba. Tourism is a critical component of Cuba’s economy that has grown in recent years as the U.S. relaxed restrictions.

Almost a year after diplomats began describing unexplained health problems, U.S investigators still don’t know what or who is behind the attacks, which have harmed at least 21 diplomats and their families, some with injuries as serious as traumatic brain injury and permanent hearing loss. Although the State Department has called them “incidents” and generally avoided deeming them attacks, officials said Friday the U.S. now has determined there were “specific attacks” on American personnel in Cuba.