BRUSSELS — Back in June, the European Union urged its member countries to reopen their borders to travelers from the United States, hoping to give a boost to the continent’s ailing tourism sector in the crucial summer season.

It worked. American tourists flocked to the beaches of Spain and Greece, the countryside of Italy and the streets of Amsterdam and Paris.

But on Monday, the European Union proposed new travel restrictions for unvaccinated visitors from the United States, a response to the alarming surge in coronavirus cases and hospitalizations across the Atlantic.

In removing the United States from a “safe list” of countries whose residents can travel without requirements such as quarantine and testing, the European Council of the European Union, which represents governments of the bloc’s 27 countries, signaled that potential restrictions to curb the spread of the coronavirus may remain in place for months. The new measures could deal a fresh blow to Europe’s ailing tourism sector.

Other countries removed from the “safe list” include Israel, Kosovo, Lebanon, Montenegro and North Macedonia, most which have reported a surge of cases in the past 14 days, according to a New York Times tracker.

The suggested restrictions are not mandatory, and it remains up to each European Union member state to follow the guidelines. So it was not immediately clear which countries, if any, would reintroduce restrictions or when they might begin.

If enforced, the new restrictions would apply to unvaccinated travelers only. The European Council already recommends that all visitors who have been inoculated with an E.U.-approved vaccine be allowed to travel. That includes the three vaccines available in the United States and manufactured by Johnson & Johnson, Pfizer and Moderna, as well as AstraZeneca.

Some countries have also implemented more stringent measures than others, even for visitors from a country on the safe list, yet once visitors have entered an E.U. country, they can move across the bloc freely.

Under the current guidelines, unvaccinated travelers from countries on the European Council’s safe list can visit E.U. countries without quarantining by showing a negative test. But a minority of countries have also kept self-isolation requirements in place, including, in some cases, for vaccinated visitors.

Meanwhile, United States has remained closed to Europeans, who have expressed frustration at the lack of reciprocity.

With more than 52 percent of Americans fully vaccinated, most were able to travel to Europe without hurdles this summer and can continue to do so. Yet the decision to remove the United States from the safe list could still create confusion among American tourists, said Marie Audren, the director of HOTREC, a lobbying group that represents the hospitality industry in Europe.

“Every client at a hotel, a restaurant, a bar or a cafe was valuable to the tourism industry this summer,” Ms. Audren said. “And in recent years American tourists have become increasingly important to European countries.”

In France, Greece and Spain, U.S. visitors make up the largest contingent of tourists from non-European countries, according to data provided by tourism ministries. In others, such as Portugal, total spending by Americans is among the highest of any nationality.

Yet U.S. arrivals to Europe declined by more than 80 percent last year compared with 2019, according to the European Travel Commission, a Brussels-based group that represents national tourism organizations on the continent. While figures for this summer aren’t available yet, Ms. Audren said it would take years to return to prepandemic levels.

Luís Araújo, the president of the European Travel Commission, said, “Further unjustified changes in regulations will undoubtedly negatively affect the tourism sector, which is slowly recovering from its worst crisis.”

In Europe, the number of coronavirus cases has remained stable this month. But the United States recorded more than 100,000 daily Covid hospitalizations over the last week, a first since the winter. E.U. officials are wary that an influx of unvaccinated U.S. visitors could drive infections up in Europe.

One of the European Council’s criteria for lifting restrictions is that a country should have fewer than 75 coronavirus cases per 100,000 inhabitants over the previous 14-day period. The United States has had a reported infection rate well above that threshold for weeks, according to data provided by the European Center for Disease Control, and it is classified as a red zone by the agency — the second-most risky classification.

A European official with knowledge of confidential discussions leading up to the announcement, who spoke on condition of anonymity because he was not authorized to discuss the update to the list, said that it had been made based on the latest scientific data available, and that the infection numbers in the United States spoke for themselves.