Human Rights Watch on Tuesday strongly criticised a 10-year prison term handed to a Kuwaiti tweeter for religious insults and called on authorities to immediately release him.
"The ruling is another example of a violation of the right to free speech in Kuwait," the New York-based HRW said in a statement.
"Ten years in prison for peaceful criticism shows just how little Kuwait respects freedom of expression," said Joe Stork, deputy Middle East director at HRW.
"Locking up critics isn’t going to make Kuwait’s political crisis go away."
Kuwait's court of appeals on Monday upheld the heavy jail term against Shiite citizen Hamad al-Naqi for remarks on Twitter deemed offensive to Islam's Prophet Mohammed, his wife and companions.
Naqi, 23, who has been in jail since March 2012, was also prosecuted for criticising the leaders of Gulf states Saudi Arabia and Bahrain.
"Authorities should quash the verdict and release al-Naqi immediately," HRW said.
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Naqi was handed the heavy sentence by a lower court in June last year. The ruling can still be challenged in the supreme court.
He posted the offensive messages on two Twitter accounts in February and March of 2012, according to the ruling.
Naqi claimed his Twitter accounts were hacked during that period.
In recent months, Kuwait has jailed tweeters and activists amid sectarian tensions between the emirate's Sunni majority and Shiite minority.
Opposition activists have also been jailed for using Twitter to insult the emir of the oil-rich Gulf state.
"As a state party to the International Covenant on Civil and Political Rights and the Arab Charter on Human Rights, Kuwait is required to protect the rights to freedom of opinion and expression," HRW said.
"Punishing Hamad al-Naqi for criticising neighbouring monarchs clearly violates international rights standards," Stork said. "It’s disappointing that on appeal the authorities didn’t try to remedy this violation."