(Refiles to restore word "not" in 4th para)

DUBAI, May 15 (Reuters) - A Bahraini court jailed six people for a year on Wednesday for insulting King Hamad bin Issa al Khalifa in messages on the microblogging site Twitter, the official news agency said.

The six were accused of writing remarks "undermining the values and traditions of Bahrain's society towards the king on Twitter," the Bahrain News Agency quoted the head of the public prosecutor's office, Nayef Youssef, as saying in a statement.

He said freedom of opinion and expression were guaranteed by the Gulf Arab island state's constitution, law and international conventions, but should not be used in a way that contradicted the norms of society.

The news agency gave no further information about the six. It was not immediately known whether they had any role in pro-democracy ferment that has shaken Bahrain, a strategic U.S. ally facing Iran across the Gulf, since early 2011.

Home to the U.S. Navy's Fifth Fleet, Bahrain has faced unrest since February of that year, pitting a Shi'ite Muslim-dominated opposition against the minority Sunni-led government, led by the Al Khalifa family.

An initial uprising was crushed, dozens of people were killed and authorities razed "Pearl Square" where the mostly Shi'ite demonstrators camped out in central Manama.

Now weekly sessions of a reconciliation process between government and opposition known as the "national dialogue" take place outside Manama. But the political atmosphere remains tense, with near nightly skirmishes between youths and police.

Shi'ites complain of discrimination in jobs and government while their loyalty in turn is questioned by members of the Sunni ruling family, bound by historical and marriage ties to neighbouring Saudi Arabia.

The government denies opposition allegations of discrimination and of arbitrary arrests and abuses by security forces. It has accused Shi'ite Iran of stoking the unrest, something the Islamic Republic has consistently denied. (Reporting by William Maclean; Editing by Mark Heinrich)

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