ANKARA - Turkish authorities suspect a high-profile British jihadist detained in Turkey last week may have been planning attacks in Istanbul similar to those in Paris, two security sources told Reuters on Sunday.A man thought to be Aine Lesley Davis, an associate of the Islamic State militant dubbed "Jihadi John", was detained in Istanbul last week, senior Turkish officials said on Friday.Davis was detained with a group of others who could have been planning an attack in Istanbul in parallel with the gun and bomb rampage in the French capital which left at least 129 people dead, a separate source said on Sunday."Davis is a figure with key responsibilities within Islamic State and he wasn't caught alone. He was within a group," the source said."Right now we're investigating whether they were planning an attack in Istanbul similar to the one in Paris. We suspect there could have been a parallel attack with Paris, on the same day."Davis is one of a group of British Islamists believed to have been assigned to guard foreign prisoners by Islamic State, alongside Mohammed Emwazi, nicknamed Jihadi John after appearing in videos showing the killings of US and British hostages. Emzawi is thought to have been killed in a drone strike in Syria, US and British officials said on Friday.Istanbul is one of the world's most popular tourist destinations, with more than 10 million people visiting sights including the Blue Mosque and Hagia Sofia each year.Separately, the source said eight people suspected of links to Islamic State were detained in Ankara on Sunday, but declined to give their nationalities.