BEIRUT — 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-Islamic State efforts in the region, as he played a key role in mediating between Arabs and Kurds and in shaping US policy in the area.

The main Kurdish party in Syria said the death 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 militants in 2015.

Alloush moved on to play a key role in forming the USbacked civil council for Raqqa, also a majority Arab town that was also the de facto capital of Islamic State. 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 the Islamic State to evacuate the city after they were squeezed onto a small sliver of land.

The deal allowed a number of militant fighters to leave Raqqa in a convoy of vehicles, sparking criticism that they were let off the hook.

The US-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 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 peace.

Those who killed him ‘‘want to incite sedition between ethnic groups and ignite internal infighting,’’ said Yousef.