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Most of Europe is still closed to American travelers, or is so complicated to enter because of coronavirus restrictions that travel would be more of a burden than a getaway.

An effort is underway to make travel between European Union countries easier for those who are vaccinated, tested or recovered from the virus — but when that could extend to the United States remains unknown.

"Recently the European Commission revealed its proposal for a Digital Green Certificate to restart intra-E.U. travel. Notably, the proposal foresees the possibility to extend the Digital Green Certificate beyond the E.U. Member States — to compatible certificates issued in other countries," Eduardo Santander, executive director of the European Travel Commission, said in an e-mail. "If the U.S. has a similar system in place, this will allow to greatly facilitate travel of American citizens to their favorite European destinations."

Amid the uncertainty, a smattering of countries have said they will allow Americans to visit now, as long as they comply with COVID entry requirements. That usually means proof of a negative test, recovery from infection or a vaccination.

There are some caveats to consider: U.S. citizens thinking of traveling abroad should be aware that they will need to get tested no more than three days before flying back and show their negative result before boarding. And the U.S. State Department recently gave its highest travel advisory — Level 4: Do Not Travel — to a significantly higher chunk of the world, including six of the countries listed below. Only Iceland is at Level 3, which means the agency recommends that people reconsider travel there.

With that in mind, here are seven countries — either technically part of Europe, members of the E.U., or straddling Europe and Asia — that are welcoming Americans.

Greece

As part of a gradual reopening, Americans are allowed to visit Greece as of April 19. A broader reopening is planned for mid-May. Visitors need either a negative PCR test within 72 hours of arrival or proof of vaccination at least 14 days before getting to the country, and they may be subject to a random coronavirus test. Anyone visiting the country has to fill out a passenger location form before arriving.

The U.S. Embassy and Consulate in Greece warns that the country is still under lockdown and movement is restricted. Those who violate rules about wearing a mask, social distancing and restricting movement are subject to fines.

Iceland

U.S. citizens can visit Iceland if they are fully vaccinated against the coronavirus or can provide proof of prior infection. Even if vaccinated or recovered, visitors will need to undergo one coronavirus test when they arrive and wait for results — which are typically available between six and 24 hours — at the place where they're staying. According to Visit Iceland, the testing rule is temporary and will be reviewed May 1. Preregistration is required.

Croatia

Travelers from the United States can visit Croatia, but they must meet dual requirements: a negative coronavirus test no older than 48 hours or proof of full vaccination or recovery and proof of paid accommodation for a hotel, rental, camp or other place to stay. The U.S. Embassy in Croatia notes that a reservation alone is not sufficient; there has to be evidence of full payment.

Visitors can also pay for a test upon arrival, but they will have to self-isolate until they have a result.

Montenegro

Americans need a negative PCR test or proof of vaccination to visit Montenegro, which still has an overnight curfew in place. Bars and restaurants are open from 7 a.m. until 11 p.m., though the number of people at each table is limited. National parks are also open, while nightclubs are closed and mass gatherings are banned. The U.S. Embassy also says some intercity travel restrictions are in place.

Serbia

U.S. citizens need a negative PCR or antigen test at least 48 hours before entering Serbia. Visitors are required to wear masks, even in open spaces, and keep a safe distance between people who aren't from the same household.

The U.S. Embassy says cafes, bars and restaurants with gardens or patios are only able to operate outdoors with limits on the number of people at each table. Cultural institutions can be open until 10 p.m.

Cyprus

The Mediterranean island west of Syria and south of Turkey is a member of the European Union. It reopened to visitors from the U.S. on April 1, as long as they have a negative coronavirus test. Visitors need to fill out a "Cyprus Flight Pass" in advance. The U.S. Embassy cautions that as of April 19, American travelers will need to get another coronavirus test upon arrival in Cyprus.

Georgia

Americans can visit the country between Turkey and Russia with proof of full vaccination. Those who are not vaccinated can only fly direct from certain countries and must have a negative coronavirus test within 72 hours, as well as a second test on the third day of their stay. The U.S. Embassy says that a curfew is in place, and there are fines for not wearing a mask.