BERLIN (AFP) - Some German victims of the suicide attack in Istanbul were on a three-city tour organised by Berlin agency Lebenslust Touristik, the company said on Wednesday (Jan 13).

The agency said it still did not know how many of its clients counted among the eight Germans killed in the attack and 15 wounded, spokesman Ingo Lessmann said.

But the company had said that to its knowledge, some of the fatalities were members of its tour group.

The group, which had arrived on Monday evening in Istanbul, include 33 people from Germany, and were due to fly on to Dubai before continuing their vacation in Abu Dhabi, Lessmann said.

Chancellor Angela Merkel said on Tuesday at least eight Germans were among the victims, and that nine others were injured.

The authorities in the western region of Hesse on Wednesday announced that a 67-year-old man was among those killed in the blast, while his 50-year-old wife was wounded.

Officials from another western state, Rhineland-Palatinate, meanwhile said two men and a woman were killed.

Turkey has been hit by a string of deadly attacks this year, but Tuesday's bombing was the first time in recent memory tourists had been targeted in the heart of the city.

Images from the scene on Tuesday showed several bloody corpses lying on the ground close to the iconic Ottoman-era Blue Mosque in Sultanahmet, a district home to Istanbul's biggest concentration of historic monuments.

The Turkish authorities on Wednesday probed how a militant from Syria carried out the attack that raised alarm over security in Istanbul.

The government said the attack was carried out by a 28-year-old Syrian man who was a member of the Islamic State in Iraq and Syria (ISIS) and had recently entered Turkey from Syria.

Turkey, which has been vehemently criticised for not doing enough to crack down on ISIS, in the last two days has arrested dozens of suspected militants, although it is not clear if the action had any link to the bombing.

Turkey has been hit by a string of deadly attacks blamed on militants in the last year.

The bomber, identified as Nabil Fadli, detonated his charge on Sultanahmet Square, which is home to Turkey's most visited historic sites including the Ottoman-era Blue Mosque and Hagia Sophia church.

The explosion went off by the Obelisk of Theodosius, a monument from ancient Egypt that was re-erected by the Roman Emperor Theodosius and is one of the city's most iconic landmarks.

Police on Wednesday removed a cordon set up after the attack which had prevented access to the the area, which was thronged by media and some tourists, an AFP correspondent said.

Some placed red roses to remember the victims by the obelisk, which appeared to have sustained no damage in the bombing.

The Sabah daily said the bomber had entered Turkey as a refugee from Syria on January 5.

He was then fingerprinted by the Turkish migration service, explaining why the authorities were able to identify the bomber so rapidly after the attack.

With Germany rocked by the attack, German Interior Minister Thomas de Maiziere is flying to Istanbul to visit the site of the attack and meet Turkish officials on Wednesday.

"The terrorists are the enemies of all free people, they are enemies of humanity, be it in Syria, Turkey or France or Germany," German Chancellor Angela Merkel said, stressing that Berlin would fight terror "with determination".

The German Foreign Ministry has advised Germans to keep away from large groups in public places and tourist attractions in Istanbul.

German tourism giant TUI said that customers who had booked trips to Istanbul can switch destination without penalties.

Fifteen people were wounded, two of them seriously. The injured were mostly Germans but also include Norwegians, Peruvians and at least one Turk.

The tourists were part of a group of 33 people who had been staying at a boutique hotel in the upmarket Galata district on the other side of the Golden Horn and had been taken by tour bus to Sultanahmet that morning, Turkish media reports said.

Turkey detained 68 suspected members of the Islamic State group in raids across the country on Tuesday and Wednesday but it was not clear if there was any connection with the Istanbul attack.

In Ankara, the authorities detained 16 people who were suspected of planning a major attack in the capital, Anatolia news agency said.

On Wednesday, three more suspected ISIS members were detained in the southern resort city of Antalya. All three are Russian citizens, it added.

Turkey has been on high alert after a series of attacks blamed on ISIS, including a double suicide bombing in October in Ankara that killed more than 100 people.

Previous attacks in 2015, including the Ankara bombings, had all targeted pro-Kurdish groups who are staunchly opposed to ISIS.

But it was the first time any recent attacks specifically targeted tourists in Turkey, whose tourism industry is a key component of the economy.

Long accused by its Western allies of not doing enough in the fight against ISIS, Turkey is now hosting aircraft from the US-led coalition engaged in deadly attacks on the jihadist group strongholds.