DUBAI: Bahrain's king on Monday approved a constitutional amendment granting military courts the right to try civilians, raising concerns among rights groups for activists in the Gulf kingdom.

The decision comes as the Sunni-ruled kingdom tightens its grip on dissent, with scores of largely Shiite activists sentenced to lengthy prison terms on charges that include insulting the state, threatening national security and "terrorism".

Bahrain, a key US ally that neighbours Saudi Arabia, has been rocked by frequent protests since authorities cracked down on Shiite-led demonstrations demanding political reforms in 2011.

Military courts were previously limited to trying members of the armed forces or other branches of the security services, and could only try civilians under a state of emergency.

Bahrain's King Hamad had declared a temporary three-month state of emergency after the crackdown on protests in 2011, allowing special courts to try civilians connected with a wave of protests.

With Monday's approval of the amendment, military courts now have the power to try any civilian accused of threatening state security.

The official BNA news agency said Hamad had approved the amendment to Article 105(b) of Bahrain's constitution.

An explanatory note tweeted by Bahraini MP Mohammed Al-Ahmed states that the amendment drops a phrase that restricted military courts to trying members of the "Bahrain Defence Forces, National Guard and Police".

Monday's move coincided with a decision by the kingdom's top court to reduce the jail sentence of the leader of main Shiite opposition faction, Sheikh Ali Salman, who had been convicted of inciting hatred and insulting the state.

Sentence cut 

Salman's prison sentence was cut from nine years to four years.

The constitutional amendment was approved weeks ago by both the 40-seat upper house of parliament, appointed by the king, and the 40-seat elected lower house.

It did not formally take effect until the king's approval on Monday.

News of the amendment has sparked harsh criticism among rights groups, who warn that military courts would mean speedier trials and harsher sentencing for civilians.

Amnesty International called the amendment a "disastrous move towards patently unfair" trials of civilians, warning that it could be used to try activists on "trumped-up charges".

Authorities have justified the move as necessary to fight what they say are Iran-linked anti-government cells that have targeted the state.

The kingdom has tightened its grip on dissent over the past six years, stripping dissidents of citizenship and banning foreign media.

Last year, the authorities ordered the dissolution of Shiite opposition group Al-Wefaq, headed by Sheikh Salman, for links to "terrorism".

Al-Wefaq had been the largest bloc in Bahrain's elected lower house of parliament.

Bahrain, home to the US Navy's Fifth Fleet, accuses the opposition of working with predominantly Shiite Iran to incite unrest in the kingdom.

Tehran has consistently denied involvement.

Reports emerged last week of US President Donald Trump's plans to lift restrictions on the sale of F-16 warplanes to Bahrain as his administration moves to strengthen ties to Manama.

Trump's predecessor Barack Obama had set stricter conditions for the sale of the supersonic jets over concerns about human rights abuses in Bahrain.