Bahrain on Monday postponed until Dec. 15 the trial of prominent democracy activist Nabeel Rajab to permit more investigation of a Twitter account he is accused of using to publish criticism of the government, his human rights group said.



Rajab's Bahrain Centre for Human Rights said the High Criminal Court postponed the trial for a fourth time to enable it to hire a cyber crime expert to verify that the Twitter handle in question was managed by him.



"The reopening of his case throws a light on the lack of evidence of any wrongdoing," a statement by the center added.



There was no immediate response from the authorities to a Reuters request for comment on the case.



Rajab, one of the most prominent rights activists in the Arab world and founder of his rights advocacy group, has been repeatedly imprisoned since pro-democracy unrest flared in the Western-allied, Gulf Arab kingdom in 2011.



The U.S. State Department on Friday reiterated a call for his release, saying it rejected the charges against Rajab and Bahrain should respect freedom of expression.



The UK-based Bahrain Institute for Rights and Democracy quoted his wife Sumaya as saying the continued detention of Rajab, in his early 50s, was very worrying for their family.



"Why do they keep him locked away? He is not going to cause harm. We are concerned about his deteriorating health," she was quoted as saying.



Rajab was arrested on June 13, nearly a year after being freed by a royal pardon from a six-month sentence handed out in May 2015 for making alleged remarks deemed insulting to the kingdom's security establishment.



He was arrested on charges related to anti-government tweets published last year, including one accusing the security forces of torturing detainees. In September 2016, prosecutors filed further charges accusing him of damaging Bahrain's reputation.



His trial has coincided with what rights groups say is an escalating crackdown on opposition groups and rights activists.




The Bahraini government denies systematic rights abuses.



Bahrain is a staunch U.S. ally and hosts the U.S. Fifth Fleet as a bulwark against Iran on the other side of the Gulf.