The German Parliament is to vote on whether to label the killings of Armenians by Ottoman Turks a century ago as genocide, a move that threatens to increase tensions with Turkey at a sensitive time.

The motion, which is backed by Chancellor Angela Merkel's conservative bloc, their partners in the government, the Social Democrats, and the opposition Greens, looks set to pass with wide support on Thursday.

Historians estimate that up to 1.5 million Armenians were killed by Ottoman Turks around the time of World War I, an event viewed by many scholars as the 20th century's first genocide.

Turkey denies that the killings that started in 1915 were genocide and contends the dead were victims of civil war and unrest. Ankara also insists the death toll has been inflated.