Less than a week after the attack in the western Turkish metropolis of Istanbul, five people have been taken into custody in Bulgaria. They are accused of "assisting the probable perpetrator of the terrorist act" that left six people dead, as Sika Mileva, spokeswoman for the Bulgarian Prosecutor General's Office, told state radio in Sofia on Saturday.

Those detained in Bulgaria were "more likely to have provided logistical and communication assistance" after the attack, Mileva said, but did not give details on how they might have reached Bulgaria. Bulgarian state television said Moldovan nationals and "people of Arab origin" were among those arrested. They would be held in police custody for 72 hours and then brought before the prosecution.

On Friday, a court in Istanbul issued 17 arrest warrants. Among them is the main suspect, Ahlam Albashir. They are accused of "destruction of state unity", "premeditated killing" and "attempted murder", the prosecution said.

Eight more detentions in connection with the attack were reported by Istanbul police on Saturday. Among them is at least one person arrested in the Turkish occupied zone of Azaz in the north of Syria. Azaz is about 50 kilometres north of the city of Aleppo and seven kilometres south of the Turkish border. The city of around 70,000 inhabitants has been occupied by Turkey and allied jihadist militias since August 2016.