Nineteen members of the US Congress have written to Bahrain's king to call for the release of prominent human rights activist Nabeel Rajab, who is currently serving a three-month jail sentence for comments made on Twitter.

Rajab, the president of the Bahrain Centre for Human Rights (BCHR), appeared in court on Sunday in a separate case that relates to him calling for, and participating in, an "illegal gathering".

The verdict in the gathering case is due on August 16, according to Said Yousif from the BCHR.

The US politicians' letter, whose signatories include Senators Patrick Leahy and Ron Wyden, and Representative Keith Ellison, along with 17 other representatives, was sent on Friday to Bahrain’s King Hamad bin Isa Al Khalifa.

The politicians acknowledge that the government has made "positive" reforms since protests calling for democracy began last year, but they also said Rajab's prosecution runs "counter to the government's assurances that individuals will not be prosecuted for peaceful political speech".

"We therefore respectfully urge the government to unconditionally and immediately release all Bahrainis being held for crimes related to freedom of expression," it concludes.

'Fabricated cases'

On Saturday, Sumaya Rajab, the wife of Nabeel Rajab, also wrote a letter to the governments of the US and the UK asking for their support.

In the letter, Rajab wrote that the government has "fabricated a number of cases" against her husband in revenge for his rights work.

"We, as the family of Nabeel Rajab, plead to you and plead to the UN and all international human rights organisations and institutions to demand from the Bahraini authorities that Nabeel Rajab should be released," she wrote.

Several other prominent rights groups, including US-based Human Rights First, have also called for his release.

A report by the state-run Bahrain News Agency said that Rajab was sentenced to three months for "insulting Muharraq [an area near the capital Manama] people on his Twitter account".

"A group of people from Muharraq lodged a complaint against the defendant [Rajab] for tarnishing their reputation and casting doubt on their patriotism," Chief Prosecutor Nayef Yusuf Mahmoud was quoted as saying.

The Gulf island's ruling monarchy is a close ally of Washington and home to the US navy's Fifth Fleet.

Source: Al Jazeera and agencies