United Airlines is adding three new flights from the United States to European markets that are beginning to reopen to vaccinated visitors, the carrier announced Monday.

Starting in July, the big US airline will offer international service to Dubrovnik, Croatia, Athens, Greece and Reykjavik, Iceland, United said.

The announcement came as Greece said it would lift its quarantine requirement for fully vaccinated visitors from the United States and five other jurisdictions.

Travellers must now present either proof that they are either fully vaccinated, or a negative Covid test from the previous 72 hours, Greece's civil aviation authority said. 

The decision also ends a week-long quarantine requirement applying to visitors from the EU, Britain, Israel, Serbia and the United Arab Emirates.

Iceland and Croatia have previously announced steps to eliminate mandatory quarantines for fully vaccinated passengers or those possessing a negative test.

The announcement is a bright spot on the international aviation front, which for US carriers is lagging the recovery seen in domestic markets.

"As countries around the world begin the process of reopening, leisure travelers are eager to take a long-awaited getaway to new international destinations," said Patrick Quayle, United's vice president of international network and alliances.