The jailed leader of Bahrain's main opposition bloc, Sheikh Ali Salman who is currently serving four years for inciting disobedience, could face new charges over messages posted on Twitter.

The Al-Wefaq chief has been "referred to the public prosecution following violations posted on his Twitter account", the authorities announced Sunday on the official BNA news agency.

Al-Wefaq denounced the move, saying it "violates the Bahraini constitution and national law, as well as international covenants related to freedom of opinion and expression".

The Shiite grouping in the Shiite-majority but Sunni-ruled Gulf kingdom said Salman's tweets "focused on the values of peace and love".

It said they shared quotes by American civil rights leader Martin Luther King "about freedom and justice".

It was unclear whether Salman was himself tweeting from prison, or how.

The opposition chief was sentenced on June 16 to four years after being convicted of inciting disobedience and hatred.

An appeals court is reviewing that conviction, but the prosecution is demanding the annulment of his acquittal on the more serious charge of plotting to overthrow the regime and seeking a tougher sentence.

A ruling on the appeal is expected on March 30.

Al-Wefaq renewed earlier calls for its leader to be released "immediately".

The group once held the most seats in parliament, but its 18 MPs walked out in 2011 in protest at violence against demonstrators during pro-democracy rallies.

Bahrain's authorities crushed Shia-led protests a month after they erupted on February 14, 2011.

The gap has since been growing between the Sunni authorities and their mainly Shia opponents.

Tiny but strategic Bahrain across the Gulf from Shiite Iran is home to the US Fifth Fleet, and on October 31, construction work also began on Britain's first permanent military base in the Middle East since 1971.