The Istanbul synagogue bombings have triggered international condemnation - and the Turkish prime minister called them an "attack against humanity".
Erdogan: "Whatever steps need to be taken ... will be taken"
Recep Tayyip Erdogan, cutting short a visit to Turkish-controlled northern Cyprus, said the bombings targeted Turkey's "stability and peace".
Neighbouring Greece said the bombings were "barbaric and atrocious".
The Israeli foreign ministry said terrorism was a global threat "not only directed against Israel or the Jews".
Foreign ministry spokesman Jonathan Peled said "we should stop turning a blind eye towards all the incitement and hatred that is coming out of certain parts of the world".
Reaction from the United States came in a statement from President George W Bush.
"I condemn in the strongest possible terms the terrorist attacks today in Istanbul, where Turkey's diverse religious communities of Muslim, Jewish, and Christian believers have
flourished together for centuries," he said.
The European Union described the bombings as "an unacceptable expression of intolerance and rejection that have to be eradicated".
European Commission President Romano Prodi visited a synagogue in Milan to express his sorrow over the attack.
In other reactions:
France said it "condemns with the greatest vigour the odious double attack", which only "strengthen the determination of democratic nations to combat together anti-Semitism and all forms of intolerance, as well as fight unceasingly against terrorism".
German Foreign Minister Joschka Fischer said the bombing of synagogues "fills us with particular horror and outrage". He stressed that the fight against international terrorism and anti-Semitism "must be a collective effort of the international community".
Italy said it was sharing with Turkey "the tragic consequences of this vile manifestation of violence" - an apparent reference to Wednesday's truck bombing in the southern Iraqi town of Nasiriya which killed 19 Italian soldiers and policemen.
Pope John Paul II urged "all men and women across the world to mobilise for peace and against terrorism".
Pakistan strongly condemned the "dastardly" bombings.