Bahrain has revoked visas for two members of Human Rights Watch who planned to attend this week's Inter-Parliamentary Union assembly in the Gulf state, the activist group said on Friday.

Visas that were issued for two members of the rights group on January 30 were cancelled on March 8, three days before the assembly for the 178-member IPU starts on Saturday, HRW said.

The IPU, which groups parliaments around the world and aims to promote democracy and human rights, said it was "aware" the visas were revoked but said it "is not responsible for the visa process which is a sovereign decision of the host country".

Bahraini authorities did not immediately reply to a request for comment.

HRW, which holds permanent observer status with the IPU, said the body should respond by urging Bahrain to "release all those imprisoned solely for peaceful speech" and rescind laws barring political opponents from contesting elections.

Authorities launched a crackdown after an Arab Spring-inspired movement of largely Shiite protesters hit the streets in 2011 to demand an elected government for the tiny Gulf kingdom of some 1.4 million people.

Hundreds of demonstrators have since been jailed and opposition parties outlawed. The government claims Iran gave the demonstrators training and support in a bid to topple it -- an accusation Tehran denies.

Tirana Hassan, HRW's acting executive director, called the visa cancellations "a blatant example of (Bahrain's) escalating repression".

"Bahrain's hosting of sporting and high-level international events is a transparent attempt to launder its decades-long campaign to crush political opposition and suffocate the country's vibrant civil society," Hassan said in a statement.

Bahrain also came under fire for its human rights record during last week's season-opening Formula One Grand Prix, when 21 rights groups raised concerns over "sportswashing", highlighting the imprisonment of political leaders.

In parliamentary elections in November, Bahrain's two main opposition groups, the Shiite Al-Wefaq and the secular Waad, were prevented from presenting candidates. These parties were dissolved in 2016 and 2017 respectively.