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.