CAIRO 

A U.S. citizen died in an Egyptian prison after being held for more than six years, Egyptian authorities confirmed Tuesday.

Moustafa Kassem was transferred to hospital on Sunday for treatment, but he died on Monday, Egypt's attorney general said in a statement.

The statement said the attorney general ordered an autopsy to determine the cause of Kassem's death. 

Earlier, the U.S.-funded Al-Hurra TV said Kassem, who was an Egyptian American, died after staging a hunger strike.

U.S. Assistant Secretary of State David Schenker expressed his sincere condolences in a briefing released by State Department.

"I'm deeply saddened to learn today of the death of U.S. citizen Moustafa Kassem, who had been imprisoned in Egypt since 2013," Schenker said. "His death in custody was needless, tragic, and avoidable."

He emphasized the State Department will continue to raise "serious" concerns over human rights and Americans detained in Egypt at every opportunity.

Kassem was arrested in 2013 following the dispersal of a major protest camp in support of former President Mohamed Morsi in Cairo's Rabaa al-Adawiya Square.

While Egyptian authorities say only 623 people were killed when security forces violently dispersed the camp, opponents put the death toll at thousands.

Morsi, who was removed from power in a military coup in 2013, died during a trial session on "espionage" charges last year. 

In September 2018, Kassem was sentenced to 15 years in prison. Shortly after his sentence, he sent a letter to U.S. President Donald Trump imploring for his help.

In December, the U.S. State Department demanded the Egyptian authorities to release him. Trump once mentioned his name to be released.

* Ahmed Asmar and Beyza Binnur Donmez contributed to this report from Ankara