President Tayyip Erdogan says Turkey will use all its military and intelligence muscle to battle "one of the biggest and bloodiest terrorist waves in its history", after a suicide bomber killed three Israelis and an Iranian in Istanbul.
Israeli Defence Minister Moshe Yaalon described Turkey as "awash in terrorism". Turkey's main opposition party blamed what it called the government's "adventure-seeking policies" in the Middle East for turmoil washing across Syria's borders.
Saturday's attack on Istiklal Street, a long pedestrian avenue lined with international stores and foreign consulates, was the fourth suicide bombing in Turkey this year. Two in Istanbul have been blamed on Islamic State, while the two others in the capital Ankara have been claimed by Kurdish militants.
The attacks have raised questions at home and among NATO allies as to whether its security services are overstretched as they fight on two fronts.
"Turkey has recently been facing one of the biggest and bloodiest terrorist waves in its history ... Our state is fighting terrorist organisations and the forces behind them with everything at its disposal - its soldiers, police, village guards and its intelligence," Erdogan said in a speech in Istanbul.
But his critics, including privately some of Turkey's allies, argue that Erdogan's focus on battling Kurdistan Workers Party (PKK) militants in the largely Kurdish southeast - a campaign he has repeatedly vowed will continue - comes at the expense of its fight against Islamic State.
Erdogan said the PKK and other groups were working with Islamic State and had turned on Turkey because they had failed to achieve their aims elsewhere in the region. He accused Europe of "two-faced behaviour" for allowing PKK sympathisers to set up a tent near an EU-Turkey summit in Brussels last week.
Turkey's Haberturk newspaper said police had been examining CCTV footage and that it appeared the suicide bomber had followed the group of Israeli tourists for several kilometres from their hotel, then waiting outside the restaurant where they ate breakfast before blowing himself up as they emerged.
Israeli media gave details of those who died.
Yonathan Suher, a father of two, had travelled to Istanbul to celebrate his 40th birthday with his wife, who was seriously wounded. Kindergarten teacher Simcha Damari, 60, and Avi Goldman, 63, who worked as a tour guide in Israel, both left behind several grandchildren, Israeli media said.
