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.