The health of a prominent activist jailed in Bahrain is failing and authorities are not providing medical care, international human rights groups said.

Nabeel Rajab - a leading figure in the 2011 protests in Bahrain - was sentenced to two years in jail last July for "disseminating rumours and false information".

He also faces a potential 15-year sentence in a second case linked to comments on Twitter that criticised Saudi Arabia and its allies, including Bahrain, over their military intervention in Yemen's war.

Bahrain's government has denied healthcare to and "detained, imprisoned, tortured... or forced into exile" a number of prominent dissidents, said a statement by the rights groups on Thursday.

The organisations said they had evidence authorities withheld medication for Rajab, who has been hospitalised multiple times in recent years, "in a clear attempt to threaten his security and integrity".

"Recently alarming signals have multiplied raising security concerns regarding his detention condition," said Dimitris Christopoulos, president of FIDH, an international consortium of 184 non-governmental organisations from around the world.

"The government should put a stop to these tactics and immediately release him. We seriously fear for his life."

A request for comment from Bahrain's Ministry of Interior wasn't immediately answered.

Bahraini authorities have jailed dozens of activists and disbanded opposition groups since 2011.

Bahrain's government accuses neighbouring Iran of backing the demonstrations in order to overthrow the government, an allegation Tehran denies.