DUBAI (Reuters) - An Iranian physicist imprisoned in Tehran since 2011 is suffering from life-threatening kidney cancer, the International Campaign for Human Rights in Iran said on Monday.

Omid Kokabee, 34, was arrested while on a visit to Iran from the United States, where he was studying physics at the University of Texas. He was charged with communicating with a hostile government and receiving illegitimate funds.

His family has asked for his immediate release due to his severe health condition.

"The prison doctors were prescribing painkiller without even examining him," an informed source was quoted as saying by the International Campaign for Human Rights in Iran, an independent organisation based in New York.

"It was only last week that Omid was diagnosed with cancer and now the cancerous tumour has spread all over his right kidney," it said.

Iranian judiciary officials have said Kokabee was convicted and sentenced to 10 years in jail and the Islamic Republic will not bow to international pressure calling for his release.

Kokabee has said he had been tortured in prison, both physically and mentally, and forced to make confessions.

In letters from Evin prison in 2011 and 2013, Kokabee wrote that his imprisonment was the result of his refusal to heed pressure by Iranian government to collaborate on a secret research project, most probably referring to Iran's nuclear programme.

"My only sin is that I am following a rare field of study and no one in Iran has my expertise," wrote Kokabee, who has worked in laser technology and atomic physics.

Iran has been seeking to enrich uranium by laser as it has the potential to reduce the cost and also to make the enrichment facility far smaller. In an accord with world powers signed last July, Tehran agreed to curb its nuclear programme in return for lifting economic sanctions.

Kokabee was awarded the Andrei Sakharov Prize in 2013 from the American Physical Society for "his courage in refusing to use his physics knowledge to work on projects that he deemed harmful to humanity, in the face of extreme physical and psychological pressure".

In 2014 in an open letter to Iran's Supreme Leader, Ayatollah Ali Khamenei, 18 physics Nobel laureates called for his immediate and unconditional release. Amnesty International has designated him a prisoner of conscience.

(Reporting by Bozorgmehr Sharafedin, Editing by Angus MacSwan)