The Syrian Observatory for Human Rights said the men were executed and their bodies hung on makeshift crucifixes in Albu Kamal in the eastern province of Deir Ezzor.

"The men surrendered in Albu Kamal because the ISIS had offered amnesty to people who fought them if they turned themselves in," Observatory director Rami Abdel Rahman told AFP.

Instead, he said, the eight opposition fighters were beheaded and then hung from crosses in a method often employed by the terrorists group.

The men had belonged to a group that had fought against both the Syrian Army and the ISIS.

Meanwhile, in the city of Deir Ezzor, the provincial capital, IS terrorists decapitated another three men, also hanging their corpses from crosses, the Observatory said.

The group said it was unclear when the executions took place, adding that two of the men were accused of collaboration with the Syrian government and the third of fighting against the ISIS.

But in the scene of fighting ISIS, The United States targeted ISIS militants on Sunday and Monday with five air strikes in Syria and nine in Iraq, according to U.S. Central Command.

The strikes in Syria hit militants' fighting positions and buildings and struck a small group of fighters near the key town of Kobani along the border with Turkey, while one strike hit near Dayr Az Zawr, it said in a statement on Monday.

In Iraq, the U.S.-led strikes destroyed vehicles and equipment and struck a small unit of fighters and a fighting position. The nine strikes hit near the cities of Baiji, Falluja and Rutba, the statement said.