A senior Kurdish official who played a key role with the United States in implementing its post-Islamic State group policy in northern Syria has been found dead in his apartment, Kurdish officials said Thursday.

Omar Alloush's death is a blow to the post-IS efforts in the region, as he played a key role in mediating between Arabs and Kurds and in shaping the U.S. policy in the area.

The main Kurdish party in Syria said Thursday the death of Alloush is under investigation, and officials suspect foul play.

Alloush was found dead in his apartment in Tal Abyad, a majority Arab town in northern Syria where he helped set up a joint Arab-Kurd administrative council after it was liberated from IS in 2015.

Alloush moved on to play a key role in forming the U.S-backed civil council for Raqqa, also a majority Arab town which was also the de-facto capital of IS. Raqqa was cleared of the militants last year after months of fighting.

Alloush and other Arab tribal leaders were instrumental in negotiating a deal with the remnants of IS to evacuate the city after they were squeezed into a small sliver of land.

The October evacuation deal allowed a number of IS fighters to leave Raqqa in a convoy of vehicles, sparking criticism that the militants were let off the hook. The U.S.-led coalition said it was not involved in the negotiations, which aimed to save lives.

Another Arab mediator who played a key role in an evacuation deal of IS militants from Tabqa, a town near Raqqa, was also found killed last month in Syria.

Top Kurdish official Fawza Yousef said Alloush's killing is a blow to joint Arab-Kurdish action and social peace following the defeat of IS.

"Omar Alloush had a key role in forming the Raqqa city council and in developing the concept of coexistence in Tal Abyad, Raqqa and Tabqa," she said, naming also another major town west of Raqqa that was recaptured from IS.

Those who killed him "want to incite sedition between ethnic groups and ignite internal infighting," she said.

Yousef accused Turkey of being behind the assassination of Alloush, saying he received threats before. Turkey views the dominant Kurdish militia, which is the U.S. partner in the fight against IS in Syria, as a terrorist group.

Ankara had sent troops into Syria to push the Kurdish militia away from its borders and prevent it from linking up areas it controls in west and east Syria. The U.S. support for the Kurdish-led forces was a reason for souring relations with Ankara.

Nadim Houry, director of the counter-terrorism program for Human Rights Watch, called Alloush's death "terrible news," describing him as a "cornerstone" of Kurdish-Arab relations in the area.

"Whoever killed him wants to destabilize the area," Houry wrote on his Twitter account.

U.S. officials had no immediate comment on Alloush's death.

Nicholas Heras, a Middle East Security Fellow at the Washington-based Center for a New American Security, said Alloush's suspected killing was a "terrible blow" to the U.S. efforts to stabilize Raqqa and therefore U.S. strategy in Syria. The Kurdish-led forces have "a big bulls eye on its back and the road ahead will not be easy."