The Islamic State (ISIS) has branded as an "apostate" a Muslim American soldier who was killed in Iraq whose story became a heated subject of debate in the U.S. presidential election.

"Beware of dying as an apostate," wrote Dabiq, ISIS' online magazine, along with a photo of Army Capt. Humayun Khan's tombstone in Arlington National Cemetery, Reuters reported.

An apostate is defined as someone who abandons his religion or political beliefs.

The magazine's latest edition also published an article, written by an unnamed "American convert in the Islamic State," that urged Muslims to fight back Western influences, migrate to ISIS-controlled lands or carry out lone attacks.

"Reject these calls to disunity and come together. Live the life of Islam, for which you have already left the path of falsehood," the militant said. ""You are behind enemy lines, able to strike them where it hurts them most."

Khan's father, Khizr Khan, spoke at the Democratic National Convention last Thursday, paying tribute to his son and criticising Republican presidential candidate Donald Trump for his proposal to temporarily ban Muslims from entering the United States. He also asked Trump if he had read the U.S. Constitution.

The younger Khan was killed in a bomb attack in Iraq in 2004.

As response, Trump questioned why the mother was not allowed to speak on stage at the Democratic convention.

"If you look at his wife, she was standing there," Trump told ABC News' George Stephanopoulos. "She had nothing to say. She probably, maybe she wasn't allowed to have anything to say. You tell me."

Trump said he has made many sacrifices for the U.S.

"I think I've made a lot of sacrifices. I work very, very hard. I've created thousands and thousands of jobs, tens of thousands of jobs, built great structures. I've had tremendous success. I think I've done a lot."

Ghazala Khan, in an op-ed piece in the Washington Post, wrote that Trump did not know anything about Islam and sacrifice.

Trump has appealed to Capitol Hill to support him in the Khan issue.

"We want to get several member statements out today on this, and would really appreciate your help," Rob Wasinger, who is working for the Trump camp, wrote in an email to U.S. Senate aides, according to Reuters.