A Bahraini appeals court on Thursday upheld a six-month prison sentence for prominent rights activist Nabeel Rajab for insulting public institutions on Twitter, the prosecution said.

Rajab, a member of Bahrain's Shia majority, was sentenced in January over comments he posted about the interior and defence ministries in the Sunni-ruled monarchy.

"It is obligatory now to serve the sentence," public prosecutor Wael Buallay said in a statement carried by the BNA state news agency, noting that Rajab has the right to take the case to the higher Court of Cassation.

In one of the messages deemed offensive, Rajab charged that many Bahrainis fighting with jihadists in Syria were former Bahraini security forces personnel who had developed Sunni extremist views while in service.

Rajab is already in detention following his arrest in April for posting comments on Twitter denouncing alleged torture in a prison where Shia activists are held.

The activist, who has led anti-government marches and heads the Bahrain Centre for Human Rights, had already served two years in jail for taking part in unauthorised protests and was freed last May.

Bahrain, home to the US Navy's Fifth Fleet, has been rocked by unrest since security forces crushed Shia-led protests in 2011 demanding a constitutional monarchy and an elected prime minister.

International rights groups have condemned the trials against opponents of the Sunni regime.