A Bahraini appeals court has ramped up Shia opposition leader Sheikh Ali Salman's jail sentence from four to nine years on charges of inciting violence to nine years, authorities and his bloc said.

Bahrain's public prosecutor said on Monday that the stiffer sentence related to "crimes of promoting change to the political system by force", according to the Sunni Muslim-ruled Gulf Arab monarchy's state news agency BNA.

Salman's Al-Wefaq Islamic Society bloc denounced the decision against him as "provocative" and said it undermined any chance of resolving the political crisis in Bahrain, which escalates tensions between Sunni and Shia Muslims.

Salman was sentenced last summer to four years in prison for inciting unrest.

Defence lawyers appealed in September, saying that prosecutors had presented as evidence excerpts of his speeches that were taken out of context.

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Prosecutors responded with their own appeal, asking the court to reverse Salman's earlier acquittal on more serious charges of seeking to overthrow the political system by force, according to the rights group Amnesty International.

The court extended his prison term as a result.

Salman, head of the Al-Wefaq Islamic Society, was arrested in December 2014 in a case that angered his followers and stirred unrest in the kingdom, which has a Shia majority.

Bahrain has experienced sporadic turmoil since a Shia-led uprising in 2011 that demanded democratic reforms and a bigger role in government.

That revolt was put down with military assistance from neighbouring Saudi Arabia.

Scores have died in periodic unrest which has flared despite the suppression of the original uprising, and efforts at dialogue between the opposition and the regime have failed.