Two bombs exploded outside the synagogues within minutes of each other in the heart of Istanbul on Saturday, causing widespread carnage.
Local television station NTV and CNN Turk reported around 23 people had been killed but officials have only confirmed about 20 deaths so far.
Aljazeera.net correspondent Jonathan Gorvett, who lives in the Sisli district of the city where the second bomb exploded, says that the death toll may rise.
''I heard the second explosion go off and saw plumes of black smoke. The synagogue is in a narrow road, the back of the building has been completely destroyed along with a house opposite the synagogue. There is complete chaos and people are in shock.''
A medic checks a victim's pulse
after the deadly blast
The emergency services have been on the scene and the police have cordoned off the Galata area, scene of the first explosion when a car packed with explosives blew up outside the Neve Shalom synagogue.
The second explosion outside the Beth Israel synagogue in the Sisli district of the city went off at 10am local time (08:00 GMT). Police have confirmed the synagogue was attacked using a car full of explosives.
Gorvett says many of the dead and injured are believed to be Muslims.
''The districts where both bombs went off are predominately Muslim areas. The Galata area is made up of lamp and furniture-making workshops, all owned by Muslims.
"Most of the shops have been damaged by the explosion and the street is covered in glass.''
The injured have been taken to the Sisili hospital where doctors are appealing for blood donations.
A Turkish group called the Great Eastern Islamic Raiders Front (IBDA-C) has claimed responsibility for the bombings.
The group wants to overthrow the secular Turkish government and replace it with an Islamist administration.
A man mourns (L) as people sift
through the rubble
The leader of the banned group is believed to be Salih Mirzabeyoglu, who was arrested and imprisoned in 1998 for plotting to overthrow the government.
Mirzabeyoglu's supporters say he is an intellectual and a visionary who has had 41 books published. They claim that he has been tortured regularly since being imprisoned.
But Gorvett says that the Turkish police are not convinced that the IBA-C are responsible for the attacks.
''The security department in Istanbul has told me it doesn't believe the IBDA is responsible for the bombings. It is pointing the finger of blame at al-Qaida. The simultaneous bombings of targets is a classic al-Qaida tactic.''
The Israeli Foreign Ministry Silvan Shalom called on the international community to fight against ''the forces of evil'' following the attacks.
The Neve Shalom synagogue was the scene of an attack by a Palestinian gunman in 1986. Twenty-two Jewish worshippers were killed.
A bomb attack on the same synagogue was carried out by Lebanese group Hizb Allah in 1992.