Egypt’s top prosecutor has ordered an autopsy to determine the cause of the death of jailed US-Egyptian citizen Mostafa Kassem on Monday.

Kassem, 64, who has been in custody since 2013, died after going on hunger strike to protest his conviction.

Egypt's public prosecutor said in a statement on Monday that Kassem died at a Cairo hospital after he had been transferred from a jail hospital the night before. He ordered an autopsy and investigation of the doctors who supervised his case to determine the cause of the death.

Interior ministry records show that Kassem was subject to a medical check in April last year which showed he was in stable condition, a security source told Ahram Online on Tuesday.

Kassem, an auto parts dealer who was born in Egypt before emigrating to the US and being naturalised, was sentenced to 15 years in prison on convictions related to a 2013 pro-Muslim Brotherhood sit-in in Cairo.

He began a liquid strike in September 2018 after his conviction of terrorism and violence-related charges including murder, sabotage, disturbing public peace, assaulting people, damaging public buildings, using force to intimidate the public and resisting police.

Egypt has previously released other Egyptian American citizens such as Aya Hijazi and her husband and Mohamed Soltan, who were detained on different charges, after they gave up their Egyptian nationality.

According to international law, defendants with dual citizenship should be prosecuted in the country in which they committed the crime, legal and constitutional expert Salah Fawzy told Ahram Online.

US Assistant Secretary of State for Near Eastern Affairs David Schenker confirmed Kassem's death to reporters at a briefing Monday, describing it as "needless, tragic and avoidable."

“I will continue to raise our serious concerns over human rights and Americans detained in Egypt at every opportunity,” he added.

In December, US Secretary of State Mike Pompeo raised concerns over press freedom, human rights and Americans detained in Egypt, including Kassem, during his meeting with Egypt’s Foreign Minister Sameh Shoukry in Washington.

A senior State Department official said Monday that it was "still premature" to talk about punitive measures with Egypt over Kassem's death, when asked about the matter at a media briefing.

"We are really concerned about this and we're going to -- we're going to talk about it, about what we're going to do," the official said. "We haven't decided yet."