Police in Turkey were on the hunt Monday for three men linked to the Islamic State terror group who investigators say plotted to unleash "sensational" attacks in crowded areas across the country, just two days after a suicide bomber killed four tourists including two Americans in Istanbul.

Haci Ali Durmaz, Yunus Durmaz and Savas Yildiz are members of an ISIS-affiliated local cell, Turkey's state-run Anadolu news agency reports. Investigators did not specify whether they believed the men were in Istanbul or another city, but they put out the alert to all of Turkey's provincial police departments.

Saturday's attack killed two Israeli-Americans, an Israeli and an Iranian in Istanbul's busiest pedestrian shopping street. Turkey has been rocked by six suicide bombings since last summer.

Also, authorities on Sunday postponed a soccer game between Istanbul rivals Galatasaray and Fenerbahce over an unspecified terror threat.

Minister Efkan Ala said the bomber on Saturday was Turkish citizen Mehmet Ozturk, who was born in 1992 in Gaziantep province, which borders Syria. He said Ozturk wasn't on any list of wanted suspects and five other people were detained as part of the investigation.

The explosion wounded dozens of others. The attack targeted Istanbul's pedestrian Istiklal Street, which is lined with shops and cafes in an area that also has government offices and foreign missions.

Turkey faces a wide array of security threats including from ultra-left radicals, Kurdish rebels demanding greater autonomy who currently are locked in battle with security forces in the southeast, and ISIS. Turkey also is a partner in the U.S.-led coalition against ISIS, and its air bases are being used to launch bombing runs against the group in neighboring Syria.

Two of the attacks this year hit the Turkish capital, Ankara. An offshoot of the banned Kurdistan Workers' Union claimed the Feb. 17 car bombing that killed 29 people and the March 13 suicide bombing that killed 37 people. On Jan. 12, an attack that Turkish authorities blamed on ISIS claimed the lives of a dozen German tourists visiting Istanbul's historic sites. That attack delivered a bitter blow to the country's vital tourism sector.

Ala said Turkey was determined to press ahead with its fight against extremist groups, but admitted it was difficult to prevent suicide attacks. "We are working so that they do not happen," the minister said.

President Recep Tayyip Erdogan also vowed to defeat groups trying to harm Turkey.

"The terror groups and the forces behind them will sooner or later in the end be beaten," Erdogan said.

On Sunday, well-wishers placed carnations and candles at the scene of the attack, with one placard reading "We are on the streets, we are not afraid of you."

Earlier, Israeli authorities raised the number of Israelis killed in the bombing to three, among them two who also hold American citizenship.

The third victim was identified Sunday as Avraham Goldman, 69, from Herzliya. The two others are Simha Damari, 60, from Dimona and Yonata Shor, 40, from Tel Aviv.

Israel's country's counterterrorism bureau upgraded the threat level in Turkey, issuing a travel advisory that recommends Israelis avoid visiting the country.

Israeli Deputy Foreign Minister Tzipi Hotovely said Israel did not believe the Istanbul bomber targeted Israelis. "It is too early to tell if this was accidentally hitting Israelis or if it was directed at Israelis who left a restaurant," she said on Army Radio.

Speaking at his weekly Cabinet meeting, Israeli Prime Minister Benjamin Netanyahu said "Terror wreaks death and destruction around the world ... The key to the moral fight against terror is to clarify that terror, the killing of innocent civilians, has no justification - not in Istanbul, not in the Ivory Coast and not in Jerusalem."

The Israelis' bodies and other Israelis wounded in the blast were being evacuated while a senior Israeli foreign ministry official headed to Istanbul for meetings with Turkish officials.

The attack came as Turkey had heightened security across the country in the run-up to the Kurdish spring festival of Newroz on March 21, which Kurds in Turkey traditionally use to assert their ethnic identity and demand greater rights.

Ala said 120,000 police and 80,000 military police were on duty during the Newroz period and more than 1,000 police checkpoints had been set up.

Pope Francis, meanwhile, sent condolences to Erdogan over the attack. The Vatican said Sunday the message noted that Francis was grieving for the casualties and "expresses his prayerful solidarity with all touched by this tragedy."

The Associated Press contributed to this report.