The inspiration, they said, is religious: They are engaged in a holy war against Sunni extremists for control of their lands, legitimized by a fatwa issued last year by Iraq’s highest-ranking Shiite cleric.

The religious overtones of the battle for Tikrit have stirred concerns of fresh sectarian conflict, which has plagued Iraq since a U.S.-led invasion toppled the dictatorship of Saddam Hussein in 2003.

Iraqi State television on Thursday broadcast a clip showing a Shiite militia commander purportedly speaking to an Islamic State militant over a two-way radio captured from a slain militant.

“We are the soldiers of God,” says the militiaman, who identifies himself as Sayed Adeeb. “We are followers of God. We are his soldiers on the ground.”

The purported Islamic State militant, who begins weeping while delivering his response, replies: “We won’t give you an inch of the land of Muslims. We will complete our martyrdom mission.”