The U.S. State Department said on Thursday it had concerns about the case of a Kuwaiti blogger on trial for allegedly insulting Kuwait's head of state and had raised the matter with the Kuwaiti government.

Separately, the sister of blogger and journalist Mohammad Abdul-Kader al-Jassem said the charges against him were political and called for his release.

Al-Jassem, who faces up to 18 years in prison if convicted according to his lawyers, was detained on May 11 after a complaint against him was issued by the office of Kuwait's Emir Sheikh Sabah al-Ahmad al-Sabah.

We are concerned about this case and have raised our concerns with the government of Kuwait, U.S. State Department spokesman P.J. Crowley told Reuters.

The ability of citizens and journalists of any country to freely and vigorously discuss, debate and critique the actions of governments does not threaten national interests, he added. It ... makes governments better and more accountable.

Al-Jassem is also accused of spreading false news that could harm Kuwait's national interests.

His trial opened on May 24 and the court banned Kuwaiti media from publishing reports about the case.

Kuwait, the world's fourth-largest oil exporter, has the freest press among Gulf Arab states but its ruler is protected from criticism by the constitution. Many writers have been fined or imprisoned for defamation.

We demand his release and the dropping of all charges against him, his sister Sabiha Abdul-Kader al-Jassem told Reuters during a visit to London. The charges are all political because of his criticism of the government.

She added: What I want to say to the world is that Mohammad Abdul-Kader al-Jassem is a lawyer and author ... and cannot in any way break the law in anything he does or writes. He practiced his right, the right we in Kuwait have been used to, freedom of expression, freedom to write, freedom to publish.

Sabiha Abdul-Kader al-Jassem said she was dismayed at the lack of tolerance of free speech in her country.

We are astonished at what is happening because we grew up not having these kinds of detentions ... we grew up with freedom of expression, she said.

Sabiha Abdul-Kader al-Jassem, a retired planning official, said her brother's criticism of the government on his website and in several books was motivated by a love of his country, not designed to campaign for a change of regime.

On his website, he criticized the ruling al-Sabah family and accused Prime Minister Nasser al-Mohammad al-Sabah of mismanagement and corruption.

In April, he was sentenced to six months in prison for defaming the prime minister, but the court suspended the sentence pending an appeal. Two other cases against him are being heard in courts.

The blogger went on a hunger strike in protest against his detention, but health concerns forced him to end it. His sister said he had open-heart surgery in 1992 and had undergone other cardiac treatment since then.

(Writing by Arshad Mohammed; Additional reporting by London newsroom; Editing by Will Dunham)