A Boeing 787 Dreamliner operated by United Airlines takes off at Los Angeles International Airport (LAX) on January 9, 2013 in Los Angeles, California.

David McNew | Getty Images

United Airlines on Monday said it is adding flights to Iceland, Croatia and Greece for the summer in hopes that relaxed entry requirements will spur demand for popular tourist destinations.

Much of continental Europe remains off-limits for most U.S. citizens, and airline executives last week were pessimistic that would change in time for summer vacations.

But United is picking a few European destinations that have already loosened travel restrictions or are expected to in the coming weeks.

“It’s been creating a pop in searches,” said Patrick Quayle, United’s vice president of international network and alliances.

The carrier is adding a Chicago-Reykjavik flight from July 1 through Oct. 3 after Iceland last month announced it would allow tourists from outside Europe to visit, without a quarantine, if they can show proof of a Covid-19 vaccine.

After the announcement from Iceland, Delta Air Lines said it would resume service to Reykjavik from New York City and Minneapolis, and would add a flight from Boston. United had previously announced service from Newark, New Jersey, to Iceland that will run from June 3 through Oct. 29.

United will also add service from Newark to Dubrovnik, Croatia, three times a week from July 8 through Oct. 3. That country allows in visitors who can show proof of a negative Covid-19 test or proof of vaccination.

Greece last month said it would open its borders in mid-May to vaccinated tourists or those who show a recent negative Covid test result, through Reuters reported last week that shift could happen as early as this week.

United said it plans to add a Washington Dulles-to-Athens flight that will operate July 1 through Oct. 3, in addition to its Newark-Athens service, which it expects to resume on June. 3.

International service is still down though more people are getting vaccinated and some travel restrictions have been lifted.

In May, international flights will account for 40% of United’s total capacity, down from 45% during the same month of 2019. Domestic leisure demand has recovered to near pre-pandemic levels, executives said recently. American Airlines and Delta say they’ll put big planes — normally used for long-haul international flights — on popular domestic routes this summer.

One bright spot internationally for United has been northern Latin America, like Mexico, Central America and the Caribbean, said Quayle. He said doesn’t expect the rest of Europe to open immediately, though United is resuming service from Newark to Milan and Rome and from Chicago to Munich and Amsterdam next month.

“I’m most optimistic about the U.K. and the U.S. creating an air bridge between the two countries,” he said.

American travelers eager to return to Europe this summer don’t have many options because of border closures and other COVID-19 restrictions, but a few destinations are opening to vaccinated visitors.

And U.S. airlines are preparing for pent-up demand for international trips by adding flights.

United Airlines early Monday said it will offer new flights to Croatia, Greece and Iceland beginning in July.  The countries are open or soon will be open to fully vaccinated travelers.

Delta Air Lines recently announced plans to begin daily service from Boston to Iceland in May and resume Iceland flights from New York’s John F. Kennedy International Airport and Minneapolis.

And American Airlines in June will add flights from New York to Athens, Greece. Outside Europe, American is starting service from New York and Miami to Israel, another country that will begin welcoming vaccinated travelers in May, starting with tourist groups.

All are in addition to the resumption of select international flights that were canceled during the pandemic and are slowly returning as more Americans are vaccinated,  COVID-19 cases shrink in many areas, and restrictions around the globe are eased. Nearly half of U.S. adults have gotten one vaccine dose and 30% are fully vaccinated, according to the Centers for Disease Control and Prevention.

“Americans want to travel, and they want to travel abroad,” Delta Air Lines President Glen Hauenstein said on the airline’s earnings call Thursday.

Delta and others airlines are also hopeful restriction-free travel will resume between the United States and the United Kingdom as part of a “travel corridor,” but those hopes have been dashed before as talks between the two countries dragged on amid an increase in COVID-19 variants.

“We’re making progress in that regard,” Delta CEO Ed Bastian said Thursday, adding that it is not clear whether vaccines will be required as part of the proposed travel corridor.

UK loosens COVID-19 lockdownThree-month lockdown ended April 12

The CDC said this month that Americans who are fully vaccinated against COVID-19 can resume travel at low risk to themselves, but the agency is still not recommending travel given rising case counts.

Vaccinated travelers still must abide by a CDC order, issued in January, requiring a negative COVID-19 test to board international flights to the United States, and should get another test three to five days after returning.

United’s new flights to Europe for summer 2021

Here are details on United’s new routes:

Newark, New Jersey-Dubrovnik, Croatia: Flights begin July 8 and will be offered three time a week through Oct. 3. United said it will be the only airline offering nonstop flights between the U.S. and Croatia this summer. American launched nonstop flights between Philadelphia and Dubrovnik in 2019 but suspended them during the pandemic and is not resuming the service this summer.

Washington Dulles-Athens: Daily flights begin July 1 and run through Oct. 1. United also plans to resume flights between Newark and Athens on June 3.

Chicago-Reykjavik, Iceland: Daily flights begin