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Human Rights Watch on Thursday slammed the decision of Bahrain's appeals court to double the jail term of Shia opposition leader Sheikh Ali Salman as a "travesty of justice".

Salman, who heads the largest opposition formation in the Sunni-ruled kingdom, had his jail sentence increased on Monday to nine years from the original four.

He had been convicted of inciting violence but the appeals court overturned an earlier acquittal of advocating regime change by force.

The appeals court "increased the sentence despite strong evidence his initial trial was unfair and the fact that two of the charges on which he had been convicted violated his right to freedom of expression", HRW said in a statement.

"Sheikh Salman is the latest casualty of Bahrain's war on dissent, but he won't be the last unless Bahrain's allies in London and Washington loudly protest this travesty of justice," said HRW's deputy Middle East director, Joe Stork.

"This level of repression will not create stability for Bahrain, but quite the opposite," he warned.

Salman was originally convicted in July 2015, drawing condemnation from rights groups as well as both the United States and Iran.

The tiny but strategic Gulf state has been shaken by unrest since it crushed a month-long, Arab Spring inspired and Shia-led uprising demanding reforms in 2011.

Despite the 2011 crackdown, protesters still frequently clash with police in villages outside the capital Manama.

Salman's al-Wefaq political association was Bahrain's largest parliamentary bloc until its 18 MPs walked out in February 2011 in protest at the use of violence against demonstrators.

Agencies contributed to this report.