In this Sept. 29, 2017 photo, a worker carries cardboard inside the compound of the United States embassy in Havana, Cuba, Friday, Sept. 29, 2017. The United States expelled 15 of Cuba's diplomats Tuesday to protest its failure to protect Americans from unexplained attacks in Havana, plunging diplomatic ties between the countries to levels unseen in years. (AP Photo/Desmond Boylan)
In this Sept. 29, 2017 photo, a worker carries cardboard inside the compound of the United States embassy in Havana, Cuba, Friday, Sept. 29, 2017. The United States expelled 15 of Cuba's diplomats Tuesday to protest its failure to protect Americans from unexplained attacks in Havana, plunging diplomatic ties between the countries to levels unseen in years. (AP Photo/Desmond Boylan) Credit: Desmond Boylan

Washington — The United States expelled 15 of Cuba’s diplomats on Tuesday to protest its failure to protect Americans from unexplained attacks in Havana, plunging diplomatic ties between the countries to levels unseen in years.

Only days ago, the U.S. and Cuba maintained dozens of diplomats in newly re-opened embassies in Havana and Washington, powerful symbols of a warming relationship between longtime foes. Now both countries are poised to cut their embassies by more than half, as invisible, unexplained attacks threaten delicate relations between the Cold War rivals.

The State Department gave Cuba’s ambassador a list on Tuesday of 15 names and ordered them out within one week, officials said, in a move that aims to “ensure equity” between each nation’s embassy staffing. Last week, the U.S. announced it was withdrawing 60 percent of its own diplomats from Havana because they might be attacked and harmed if they stay.

The dual moves marked a sharp escalation in the U.S. response to attacks that began nearly a year ago and yet remain unexplained despite harming at least 22 Americans — including a new victim identified this week.

Still, U.S. officials emphasized they were not accusing Cuba of either culpability or complicity, merely a failure to stop whatever is happening to Americans working out of the U.S. Embassy in Havana.

Investigators have explored the possibility of a “sonic attack” harming diplomats through sound waves, but have discovered no device and identified no culprit.

“We continue to maintain diplomatic relations with Cuba, and will continue to cooperate with Cuba as we pursue the investigation into these attacks,” Secretary of State Rex Tillerson said.

Havana blasted the U.S. order, calling it “reckless” and “hasty.” Days earlier, Cuban Foreign Minister Bruno Rodriguez had pleaded with Tillerson not to take such a step. His appeals unsuccessful, Rodriguez called a news conference in the Cuban capital to again deny involvement and defend his country’s efforts to assist in the U.S. investigation.

“The Ministry of Foreign Affairs strongly protests and condemns this unfounded and unacceptable decision as well as the pretext used to justify it,” Rodriguez said.

He did not announce any retaliatory measures.

The scope of the attacks has continued to grow. The U.S. disclosed on Tuesday that a 22nd victim was confirmed the day before. In recent weeks, the State Department had said there were 21 individuals “medically confirmed” to be affected by attacks that harmed their hearing, cognition, balance and vision, some with diagnoses as serious as brain injury.