A legal team including Amal Clooney has lost its appeal before Europe's top human rights court to uphold a conviction against a man for denying the 1915 Armenian genocide.

Dogu Perincek was convicted by a Swiss court in 2005 for calling the genocide a "lie." He argued his right to free speech was violated and was exonerated by the Strasbourg-based European Court of Human Rights in December 2013.

The court on Thursday dismissed the appeal to overturn that ruling.

Historians estimate up to 1.5 million Armenians were killed by Ottoman Turks around the time of World War I, an event widely viewed by scholars as the first genocide of the 20th century. Turkey, however, denies the deaths constitute genocide, saying that those killed were victims of civil war and unrest.