Tourists look on from the balcony of a hotel in Havana, Cuba November 19, 2015. [Photo/Agencies]

The United States and Cuba have reached "an understanding" on restoring commercial airline flights.

A US State Department official said the two countries were making progress but still negotiating. An agreement has been expected before the end of the year.

The report comes on the eve of the first anniversary of the countries' historic decision to normalize relations after half a century of confrontation.

Scheduled commercial flight service has been suspended for decades as a result of Cold War animosity, although charter flights connected the countries in the interim.

The news will pave the way for US airlines to sell tickets to Cuba on their websites directly to travelers. However, only certain US citizens will be able to take up the offer because a ban on general tourism to Cuba remains in place.