DUBAI: A Bahrain court more than doubled a jail sentence against opposition leader Sheikh Ali Salman Monday, in a ruling his bloc warned risked stoking fresh unrest among the Sunni-ruled kingdom's Shia majority.

The appeals court increased the sentence for charges of inciting violence to nine years from the original four, a judicial official said.

Salman's Al-Wefaq bloc condemned the verdict as “unacceptable and provocative”, warning that it “entrenches the exacerbating political crisis” in Bahrain.

Human rights group Amnesty International issued a statement denouncing the verdict as “clearly” politically motivated.

And Britain's visiting Foreign Secretary Philip Hammond tweeted: “Raised Sheikh Ali Salman sentence in Bahrain today. Understand there is a further stage in the legal process -- will follow case closely.”

The 50-year-old Salman was originally convicted in July 2015, drawing condemnation from rights groups as well as both the United States and Iran.

Demonstrators have taken to the streets demanding his release.

Arrested in December 2014, he was also convicted last year of inciting hatred in the kingdom but acquitted of seeking to overthrow the monarchy and change the political system.

The court reversed the earlier acquittal, convicting Salman of “calling for a regime change by force”, according to a prosecution statement.

“This verdict says the ruling family has no interest in dialogue, sharing power or recognising any views other than its own,” Human Rights First said in a statement.

“Keeping the leader of the main opposition group in jail does nothing to end Bahrain's political crisis and everything to stoke further instability in the kingdom,” the US-based watchdog said.

It described the verdict as a “dangerous move” by Bahraini authorities.

Bahrain's “allies in Washington and London should be alarmed at today's decision, and at the direction the ruling family is taking the country,” it added.

'Clearly politically motivated'

Amnesty International called for Salman's release.

“Salman's conviction is clearly politically motivated and is designed to send a message to others that even legitimate and peaceful demands for reform will not go unpunished,” said James Lynch, deputy director of Amnesty's Middle East and North Africa programme.

“He is a prisoner of conscience and should never have been put on trial in the first place. He must be immediately and unconditionally released,” he said in a statement.

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

It said the verdict “reflects the Bahraini regime's settlement to reject national reconciliation by turning its back on international calls urging to address the country's political crisis”.

The ruling also “further extends the political crisis in Bahrain amid an absence of national consensus and widening human rights abuses,” it said, pledging to continue to call for “inclusive political reform”.

The tiny but strategic Gulf state has been shaken by unrest since it crushed a month-long uprising demanding reforms in 2011.

The Shia-majority kingdom, connected to Saudi Arabia by a causeway, lies across the Gulf from Iran and is home to the US Fifth Fleet.

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

Five years after the revolt, Bahrain is locked in a political impasse.

After the Arab Spring touched the small Gulf state on February 14, 2011, Bahrain's Shias demanded a more representative government and a constitutional monarchy.

Scores have died in periodic unrest despite the suppression of the original uprising, and efforts at dialogue have failed.

The government denies discriminating against Shias and regularly accuses Iran -- 200 kilometres across the Gulf -- of meddling in Bahrain's internal affairs.

Manama also frequently announces the dismantling of “terrorist” cells it says are linked to Iran, a charge that Iran denies.