Germany condemns the massacre a century ago of 1.5 million Armenians by Ottoman forces as a "genocide", President Joachim Gauck says, while adding that Germany bore partial blame for the bloodletting.

Gauck's speech at an event commemorating the centenary marked the first time that Berlin has officially used the word "genocide" to describe the killings during World War I, and an unusually strong acknowledgement of the then German empire's role in them.

"In this case we Germans must come to terms with the past regarding our shared responsibility, possibly shared guilt, for the genocide against the Armenians," he said at an ecumenical service in Berlin.

Armenians say up to 1.5 million of their kin were killed between 1915 and 1917 as the Ottoman Empire was falling apart and have long sought to win international recognition of the massacres as genocide.

Modern Turkey, the successor state to the Ottomans, rejects the claim, arguing that 300,000 to 500,000 Armenians and as many Turks died in civil strife when Armenians rose up against their Ottoman rulers and sided with invading Russian troops.

Gauck, a Protestant pastor and former East German dissident, is the head of state and serves as a kind of moral arbiter for the nation.

His statement was expected to draw an angry reaction from Ankara, which has close defence and trade ties with Berlin.

Turkish Foreign Minister Mevlut Cavusoglu said earlier on Thursday that a decision by Austrian MPs this week to condemn the massacre as "genocide" would have "unfavourable repercussions" for bilateral ties.

Ankara on Wednesday recalled its ambassador to Austria in response to the MPs' condemnation.

In his speech at the Berlin Cathedral, Gauck said that the German empire, then allied with the Ottomans, deployed soldiers who took part in "planning and, in part, carrying out the deportations".

German diplomats and observers who reported back to Berlin the atrocities they witnessed were "ignored" for fear of jeopardising relations with the Ottomans, he said.