Rights groups urged Bahrain's Western allies on Friday to pressure the kingdom to drop charges against prominent activist Nabeel Rajab, on trial over tweets deemed insulting to public institutions.

Rajab, a member of Bahrain's Shia majority which has held protests against the Gulf kingdom's Sunni rulers since 2011, was released from custody in November and will stand trial on January 20.

"Bahrain's allies including the United Kingdom, Germany, France and other European countries should publicly call on Manama to drop charges against the human rights advocate Nabeel Rajab," Human Rights Watch and the Gulf Center for Human Rights (GCHR) said.

Rajab was arrested in October after posting comments on Twitter about the interior and defence ministries.

In one of his tweets, Rajab charged that many Bahrainis fighting with jihadists in Syria were former security forces personnel who had developed extremist views while in service.

Bahrain, home to the US Fifth Fleet, is among the countries that have joined the international coalition against the Islamic State group in Syria and Iraq.

"This is an open-and-shut freedom of expression case," said HRW deputy MENA director Joe Stork. "Due process is not the issue here -- Nabeel Rajab should never have been charged in the first place."

If convicted, Rajab could face six years in prison, according to the groups.

"All those governments that are proclaiming their dedication to free expression should be making clear to Bahrain that Nabeel should not be jailed," said Khalid Ibrahim, director of programmes at the GCHR, a regional rights watchdog.

Rajab is the director of the Bahrain Centre for Human Rights and was freed in May last year after serving two years in jail for taking part in unauthorised protests.

He had led anti-government marches following a bloody crackdown on Shia-led demonstrations against the Al-Khalifa ruling family in March 2011.