Turkish authorities suspect a high-profile British Daesh extremist detained in Turkey last week may have been planning attacks in Istanbul similar to Friday's attacks in Paris, two security sources told Reuters on Sunday.A man thought to be Aine Lesley Davis, an associate of Daesh 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 that left at least 129 people dead, a separate source said on Sunday."Davis is a figure with key responsibilities within Daesh 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 extremists believed to have been assigned to guard foreign prisoners by Daesh, alongside Mohammed Emwazi, nicknamed Jihadi John after appearing in videos showing the killings of U.S. and British hostages. Emzawi is thought to have been killed in a drone strike in Syria, U.S. and British officials said on Friday.Separately, the source said eight people suspected of links to Daesh had been detained in Ankara on Sunday, but declined to give their nationalities."Daesh is also changing methods and is not only carrying out attacks inside Syria. It is showing more presence here and we are therefore taking more notice of it and responding," a Turkish official has said.Last month, the Turkish capital was rocked by a double suicide bombing at a pro-Kurdish peace rally that killed more than 100 people. It was the worst attack of its kind in Turkish history and was carried out by Turkish nationals with links to Daesh, Turkish officials have said.In July, the Turkish government had stepped up its fight against the militants, launching air strikes and opening its air bases to the U.S.-led coalition, a move that increased the risk of reprisal attacks.A senior government source said Turkey had detained more than 1,000 people suspected of having links with Daesh this year, and that 300 of them now face court proceedings.