Members of the Syrian armed group Liwa Thuwar al-Raqqa hold positions against the Islamic State outside Ayn al-Issa, in the countryside of Raqqa province on Oct. 17. (Alice Martins/The Washington Post)

A prominent member of a secretive monitoring group in Syria that works to expose Islamic State atrocities in the war-ravaged country was found dead Friday along with a friend in southern Turkey, activists said.

Members of the group, known as Raqqa Is Being Slaughtered Silently, accused the Islamic State of killing one of its most high-­profile activists, Ibrahim Abdul Qader. He and another Syrian activist identified as Fares Hamadi were found shot and beheaded in their apartment in the Turkish city of Sanliurfa, said Abu Ibrahim al-Raqqawi, a nom de guerre of a founder of the monitoring group.

The group won an International Press Freedom Award this year for providing foreign media with credible news while enduring extraordinarily dangerous conditions.

The killings, if carried out by the Islamic State, would be more evidence of far-reaching intelligence and smuggling networks run by the extremist group in Turkey and possibly other countries that neighbor Syria, analysts said.

Twitter accounts affiliated with the Islamic State praised the killings Friday.

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“We just discovered that they were dead today. They were beheaded and they had bullet holes in each of their heads,” the monitoring group’s Raqqawi, speaking by Skype, said of the activists.

He said that Abdul Qader, who was a teenager, and Hamadi were living together in an apartment in Sanliurfa — a city about 100 miles from the Syrian city of Raqqa, the self-declared capital of the Islamic State — with another unidentified person whose whereabouts are unknown. Raqqawi accused the Islamic State of carrying out the killings and intensifying its pursuit of members of the monitoring group who live outside Syria.

The undercover network of activists working for Raqqa Is Being Slaughtered Silently has become a rare and leading source of news on the Islamic State’s activities in Raqqa, a city of about 200,000 in northeastern Syria. The activists have exposed themselves to great risk to document the beheadings, floggings and other brutal punishments carried out there by militants with the Islamic State, which controls vast territory spanning Syria and Iraq.

Since it took control of Raqqa in January 2014, the Islamic State has killed members of the activist group, which also documents airstrikes in the area conducted separately by the U.S.-led coalition and the Syrian government. The group’s members operate with the utmost secrecy, with some having relocated to Turkey for fear of attack.

Raqqawi said that Abdul Qader was one of a few of the group’s members to forgo the use of a pseudonym. He chose to operate using his real identity to help the group maintain relations with the media, said Raqqawi, who added that the other slain activist was a former member of the monitoring group.

The killing of Abdul Qader raises more concerns about the Islamic State’s reach beyond its strongholds in Syria and Iraq, said Hassan Hassan, an expert on the militant group and a London-based analyst at Chatham House, a think tank.

Islamic State militants have long operated on the Turkish side of the Syrian border, using the area as a base for such activities as smuggling weapons and fighters. The group also is suspected of having plotted two deadly suicide bombings targeting peace activists in the past few months.

“This just further demonstrates that southern Turkey is no longer safe, and it’s been like that for a while,” Hassan said.

In July, Turkey and the United States reached an agreement to allow U.S. warplanes to use Incirlik air base in southern Turkey to strike Islamic State targets in Syria. U.S. aircraft have been carrying out attacks against the militants in Iraq and Syria for more than a year.

Last month, Russia entered the Syrian war, saying that it would fight the Islamic State and carrying out attacks that have bolstered Syria’s government against rebel forces.

Since Russia’s intervention, the Islamic State has made territorial gains in the strategic Aleppo province.

Cunningham reported from Cairo.

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