The city of Raqa became the first provincial capital to fall from regime control in March 2013 when an array of rebel groups seized it.

The beheaded bodies of a Syrian activist opposed to the Islamic State of Iraq and Syria (ISIS) jihadist group and a friend were found early Friday in the southern Turkish city of Sanliurfa, his colleagues said.

Ibrahim Abdul Qader, 20, and friend Fares Hamadi “were found beheaded at the friend’s house this morning,” Abu Mohammad, a founder of the activist’s “Raqa Is Being Slaughtered Silently” group, told AFP via the Internet.

The group, which documents abuses in areas under ISIS control in Syria, accused the jihadist organisation of the murders on its Facebook page.

According to Abu Mohammad, both men were from Raqa city, the de facto capital of ISIS in Syria. Hamadi was also in his early twenties.

Abdul Qader had escaped to Turkey a little over a year ago.

Members of the activist group had been killed inside Syria in the past, but this is the first time a member has been killed outside the country, Abu Mohammad added.

Turkey’s Dohan news agency reported Friday that “two Syrian journalists were beheaded” in Sanliurfa, and that seven Syrians had been arrested by Turkish police.

Sanliurfa is 55 kilometres (35 miles) from Turkey’s border with Syria’s Raqa province, a major ISIS stronghold in the country.

Turkey has long been accused by Syrian opposition activists, Kurdish fighters and sometimes even Western partners of allowing ISIS members to slip back and forth across its 911-kilometre (566-mile) frontier with Syria.

Bloody bomb attacks in southern Turkey, including an attack in July that claimed 32 lives in Suruc, have been blamed on ISIS, though the group has never claimed responsibility for the blasts.

The Raqa is Being Slaughtered Silently (RBSS) group was formed in April 2014, not long after ISIS fighters seized control of Raqa from other opposition groups.

The city became the first provincial capital to fall from regime control in March 2013 when an array of rebel groups seized it.

But since ISIS forced those groups out, it has tightly controlled the city, with activists providing a rare source of information about life in the city and surrounding province.

RBSS regularly publishes information, photos and videos about ISIS dictates and behaviour in Raqa, from a jihadist ban on private Internet connections to its decision to begin issuing identity cards.

Activists documenting ISIS abuses have regularly been targeted by the group, which deems them “infidels” punishable by death.

Shortly after the group was formed, ISIS carried out a campaign of arrests in Raqa, rounding up dozens of people.

At the time, the group’s members insisted they would continue their work nonetheless.

“It’s extremely dangerous to oppose ISIS… but we need to break the wall of fear,” Abu Ibrahim, a founder, told AFP in April 2014.

“We must make sacrifices, or else they will rule us for good, and that’s just unacceptable.”