At least 20 people were killed and more than 250 wounded on Saturday when car bombers shattered two Istanbul synagogues as worshippers celebrated the Sabbath.
Turkish officials said al Qaeda might have had a hand in it.
"It is clear that this is a terrorist event with international links," Foreign Minister Abdullah Gul said as emergency services struggled to treat those caught up in the blasts, which wrecked cars and buildings over wide areas.
A militant Turkish Islamic group claimed responsibility for the blasts, the semi-official Anatolia news agency reported.
One explosion went off outside the Neve Shalom synagogue, the city's largest. The other severely damaged the Beth Israel synagogue in the affluent district of Sisli, 3 miles away, where members of the city's tiny Jewish and Armenian, Greek communities live.
Istanbul health authorities said 20 people had been killed and 257 wounded in the two attacks.
Sabri Yalim, the head of Istanbul's fire department, told private NTV television that the scene outside the Neve Shalom synagogue looked like a war zone. He said: "There is a huge pit on the ground. The houses and cars are completely destroyed, as if a huge earthquake hit the area."
The explosions occurred during morning Sabbath prayers. Most of the casualties appeared to be residents living nearby.
NTV television said a car was seen parked just before the explosion in front of the Neve Shalom.
In a telephone call to Anatolia, a person claiming to be from the Great Eastern Islamic Raiders' Front said the militant group was responsible for the attacks, and promised more.
The caller said "the attacks would continue in the future and the reason was to prevent the oppression against Muslims," the agency said.
AP