Nearly all travel from Europe to the United States will be suspended for 30 days in an effort to slow the spread of the coronavirus, President Trump said in an address to the nation Wednesday night.

The ban, which takes effect at midnight Friday, will not include Britain.

“I am confident that by counting and continuing to take these tough measures, we will significantly reduce the threat to our citizens and we will ultimately and expeditiously defeat this virus,” Trump said.

Trump also criticized Europe for not acting to quickly enact travel restrictions for China, as the United States did, saying virus clusters here were “seeded by travel from Europe.”

The announcement came on the same day the World Health Organization declared the coronavirus a pandemic, reflecting alarm that countries aren’t working quickly and aggressively enough to fight covid-19, the disease caused by the virus. WHO officials said countries should take a “blended” approach — focusing on containment to slow the spread, in hopes of buying time for mitigation strategies.

Worldwide, there are now more than 121,000 cases of the coronavirus in 114 countries. More than 4,300 people have died. In the United States, there are more than 1,000 cases, and more than 30 people have died.

Earlier in the day, the administration had signaled that the growing number of infections in Europe was prompting it to reexamine its strategy for containing the virus, but Trump had not been expected to announce a ban.

At a hearing before the House Oversight and Reform Committee on Wednesday, Robert Redfield, director of the Centers for Disease Control and Prevention, confirmed that the administration was shifting its focus to Europe.

“A real threat right now is Europe,” he told committee members. “That’s where the cases are coming in. Europe is the new China.”

However, some experts question whether Trump’s strategy is the right approach.

Such bans can cause people to keep their travel surreptitious, making it harder to do crucial contact tracing of those who are infected. They can also disrupt the movement of health workers, experts and medical supplies. The bans, experts also point out, can cause friction, hampering information-sharing and international efforts — as has happened between the United States and China — at a time when coordination and transparency have been crucial to fighting the virus.

In Italy, where the number of infections has grown exponentially, more than 10,000 cases have been reported and there have been more than 600 deaths. This week, the country’s leader took the drastic steps of imposing a nationwide lockdown that will limit the movement of 60 million people in hopes of containing the spread of the virus. On Wednesday, the country announced even more drastic measures, saying it will stop almost all commercial activity aside from supermarkets and pharmacies.

In the United States, Trump’s announcement could also further hobble the airline industry, which is already facing enormous, potentially crippling financial pressures.

This week, Delta Air Lines, for example, announced it may cut as much as a quarter of its international flights and reduce domestic routes by as much as 15 percent due to decreased demand. The carrier has instituted a hiring freeze and is suspending its stock repurchase program. American and United are also cutting back some routes, while Southwest Airlines chief executive Gary Kelly has said he will take a 10 percent pay cut.

Carriers, including United, American and Southwest, are already dealing with fallout from the grounding of Boeing’s 737 Max following two fatal crashes that killed 346 people.

“We are in contact with the federal government to understand and comply with this directive,” American said in a statement. “The health and safety of our customers and team members remains our highest priority.”

Even if they are in place for only 30 days, the travel restrictions could cause additional damage not just to airlines but also to airports and other businesses that depend on the aviation sector.

In the early days of the coronavirus outbreak, Trump credited sweeping travel restrictions imposed on China by his administration for helping slow the spread of the virus in the United States. In that instance, all flights from China were funneled through one of 11 U.S. airports. Once there, U.S. citizens who had been in China in the past 14 days were required to undergo enhanced screening and were subject to quarantine if they showed signs of the virus; most non-U. S. citizens were barred from entering the country.

“The European Union failed to take the same precautions and restrict travel from China and other hotspots. As a result, a large number of new clusters in the United States were seeded by travelers from Europe,” Trump said.

The president also offered cautions for older Americans, who are most vulnerable to the virus, urging them to “avoid nonessential travel in crowded areas.”

Jeff Stein, Nick Miroff and Felicia Sonmez contributed to this report.