Two activists who campaigned against Islamic State in Syria have been shot in the head and beheaded in the southeastern Turkish town of Sanliurfa, the founder of the group they worked for said.

Turkey has been forced to confront Islamic State elements operating within its borders, after two suicide bombers, suspected members of an Islamic State cell, killed more than 100 people in an attack in Ankara.

Both men killed in Sanliurfa worked for Raqqa Is Being Slaughtered Silently, a campaign group founded in April 2014.

Ibrahim Abd al-Qader, born in 1998, and his friend Fares Hammadi, who had resigned from the group in 2014 because of the dangers of the work, were found dead in their apartment on Friday morning, Abu Ibrahim Raqqawi, the 23-year-old founder of the group told Reuters.

"This morning they found them in their apartment. First they shot them in the head and then they cut their heads off," Raqqawi said.

Speaking over Skype and declining to disclose his location, Raqqawi said Islamic State had previously killed one of his colleagues and also the father of an administrator in Syria, posting videos of their murders online.

He said the violence will not stop his group: "We will never stop - we will continue. They have killed a lot of our guys from the beginning. But we will not stop."

The group says it tries to fill an information void left after Islamic State cut the Internet in Raqqa. It says journalists there were either exiled or killed.

Aside from fully controlling the information flow from Raqqa, Islamic State strictly regulates public life there. It runs nearly everything from bakeries to schools, courts and mosques.

(Reporting by Dasha Afanasieva, Editing by Jonny Hogg and Alison Williams)