"Freedom of expression is one of the cornerstones of our democratic systems and we shall never relinquish it. But there are no rights without responsibility and respect for different sensibilities," the two wrote in an article in the International Herald Tribune.

"The publication of these caricatures may be perfectly legal, but it is not indifferent and thus ought to be rejected from a moral and political standpoint," they maintained.

Erdogan and Zapatero are championing a joint initiative to promote East-West dialogue, called "Alliance of Civilizations" and backed by the United Nations.

"We shall all be the losers if we fail to immediately defuse this situation, which can only leave a trail of mistrust and misunderstanding between both sides in its wake," they wrote.

"Therefore, it is necessary to make an appeal for respect and calm, and let the voice of reason be heard."