Turkey said Wednesday it had arrested a total of five suspects over a deadly suicide bombing carried out by an Islamic State (IS) jihadist that killed 10 German tourists in the historic centre of Istanbul.
Ankara has said that Tuesday's strike was perpetrated by a 28-year-old Syrian who belonged to the IS group and had recently entered Turkey from Syria as a migrant.
Prime Minister Ahmet Davutoglu said that four suspects had been arrested Wednesday in relation to the attack.
Interior Minister Efkan Ala said earlier that one person had already been arrested on Tuesday in connection with the bloodshed.
Turkish security forces over the last days also rounded up 74 suspected IS members across the country, state media said, but it was not clear if any were directly connected to the Istanbul bombing.
Davutoglu said that the bomber had not been on any wanted lists, but was registered entering Turkey from Syria as one of over 2.2 million Syrian refugees fleeing their nation's almost five-year civil war.
"This person was not being followed (as a wanted suspect). This person entered Turkey as an ordinary migrant," said Davutoglu. "All his links will be worked out now."
"Turkey is working to find the true actors behind this attack where Daesh (IS) was used as a subcontractor," he added, without explaining further.
The bomber, identified as Nabil Fadli, detonated his charge in Sultanahmet Square, 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, one of the city's most iconic landmarks.
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, but the authorities insisted he had not been on any wanted list.
The Hurriyet daily said Turkey's spy agency had twice issued warnings over the risk of a suicide attack in Istanbul.
- 'Attack on humanity' -
The German foreign ministry said 10 of those killed in the attack were German, without specifying if they accounted for all the victims.
German Interior Minister Thomas de Maiziere, who visited the wounded in hospital in Istanbul, said there was "no indication" the attack specifically targeted Germans, saying there was no need to cancel travel plans.
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"It was an attack against humanity," he said. "I see no reason to refrain from trips to Turkey."
But the German foreign ministry has advised its nationals to keep away from large groups in public places and tourist attractions in Istanbul.
German tourism giant TUI said customers who had booked trips to Istanbul can switch destination without paying a penalty.
Turkey has been hit by a string of deadly attacks blamed on jihadists over the last year, including a double suicide bombing in October in Ankara that killed more than 100 people.
But Tuesday's bombing was the first time in recent memory tourists had been targeted the heart of Istanbul.
Police on Wednesday removed a cordon preventing access to the area of the attack, which was quickly thronged by media and some tourists, an AFP correspondent said.
Davutoglu, his wife Sare and de Maiziere placed red roses by the obelisk, which appeared to have sustained no damage in the bombing.
Others wrapped football scarves around the railings from popular German teams including Bayern Munich and Borussia Dortmund, an AFP correspondent said.
The blast left 15 people wounded, most of them Germans but also Norwegians, Peruvians and at least one Turk. Berlin said seven injured Germans were being treated in hospital, five of them in intensive care.
The tourists were part of a group of 33 who had been staying at a boutique hotel in the upmarket Galata district and had been bussed to Sultanahmet that morning, media reports said.
"I saw the young man pull the pin and I shouted 'run!' in German. Then we started to run away, and the bomb instantly exploded," the group's tour guide Sibel Satiroglu told investigators, the Hurriyet newspaper said.
- Crackdown on IS -
Long accused of failing to crack down on IS, Turkey has in recent months moved against jihadist cells operating on its territory, making hundreds of arrests.
Among the 74 arrested over the last two days were 16 people suspected of planning a major attack in Ankara, Anatolia news agency said.
On Wednesday, three more suspected IS members were detained in the southern resort city of Antalya. All three are Russian citizens, it added.
In the major southern hub of Adana, 17 people were arrested, including three more Russians as well as Tajiks, Afghans and a Swedish citizen.