The boyfriend of Kayla Mueller has said he tried to save the hostage's life by pretending they were married, but she refused to lie to her Islamic State captors.

Omar Alkhani, a Syrian photographer, said he had persuaded a number of people to let him plead for her release, and posed as her husband.

But the 26-year-old denied being his wife, possibly foiling her own rescue.

"Since she's American, they would not let her go anyway," Mr Alkhani told the AP news agency.

"Maybe she wanted to save me. Maybe she didn't know I came back to save her."

The guards of her prison cell told Ms Mueller that Mr Alkhani would not be harmed if she told the truth, so she apparently stuck to honesty to save him rather than take the slim chance to save herself, he said.

Ms Mueller's death was confirmed last week by her family and the US government, though the circumstances remain unclear.

Mr Alkhani met the Arizona woman in 2010.

They were taken hostage in August 2013 after leaving a Doctors Without Borders hospital in Aleppo, Syria. He had been hired to fix the Internet connection, and she had persuaded him to let her tag along.

She wanted to see the suffering first-hand and help, despite the dangers of traveling into the war-torn region, Mr Alkhani said.

"In the end, I was afraid if she didn't go with me, she would go with someone else," he said.

Ms Mueller remained a hostage, while Mr Alkhani said he was released 20 days later after being beaten and interrogated about his work as a photographer, his religion and his relationship to the American woman.

He said he returned to Syria from Turkey later in 2013 to try to win her release, and that’s when he came up with the plan to pretend they were married.