A court in Bahrain has sentenced prominent human rights activist Nabeel Rajab to two years in jail after finding him guilty of spreading "fake news", a judicial source said.

The source, quoted by the official BNA news agency, said on Monday that Rajab was convicted "of disseminating false news, statements and rumours about the internal situation of the kingdom that would undermine its prestige and status".

The verdict, which the source said can be appealed, was immediately condemned by Bahraini and international human rights watchdogs.

WATCH: The battle for Bahrain

Rajab, 52, was sentenced in absentia as he has been hospitalised since April, said the Bahrain Institute for Rights and Democracy.

It denounced the verdict, saying Rajab had been sentenced "for speaking to journalists" and that he had "spent over a year in pre-trial detention, largely in solitary confinement".

"His lawyers state he was denied basic fair trial guarantees," the group said in a statement.

The judicial source said, however, that "all legal guarantees" were provided to Rajab and that his lawyer was present for the proceedings.

Sayed Ahmed Alwadaei of the Bahrain Institute strongly condemned the ruling.

"This outrageous sentence against someone speaking the truth exhibits the brutality of the Bahraini government and its heinous crimes and that of its kangaroo court," he said.

'Chokehold on freedom of expression'

Amnesty International also denounced the decision, saying it exposes a "relentless campaign" by authorities in Bahrain "to wipe out dissent".

"Imprisoning Nabeel Rajab simply for sharing his opinion is a flagrant violation of human rights, and an alarming sign that the Bahraini authorities will go to any length to silence criticism," said Amnesty's Secretary-General Salil Shetty.

"Bahrain's government and judiciary have once again tightened their chokehold on freedom of expression and branded him a criminal," Shetty was quoted as saying in a statement.

The US state department has expressed concern over Rajab's arrest and curbs on freedom of expression in Bahrain.

"We believe societies are strengthened, not threatened, by expressions of opinion and dissent, and that opposition voices can play a vital role helping societies become more tolerant and inclusive," read a statement released by the US state department on Monday.

The US statement called for Rajab's immediate release.

"We have repeatedly expressed our concern about Nabeel Rajab's case, and we continue to strongly urge the government of Bahrain to abide by its international obligations and commitments to respect human rights and fundamental freedoms, including the freedom of expression," added the statement.

Bahrain, however, is also a close ally of the United States and hosts the Navy's 5th Fleet. President Donald Trump has previously touted Washington's "wonderful relationship" with Bahrain.

Rajab had been arrested multiple times in recent years over protests that the government said were unauthorised.

The activist was pardoned for health reasons in 2015 before being rearrested in June 2016.

He also faces trial for a series of tweets criticising a Saudi-led military campaign in Yemen, with a hearing due on August 7, according to the Bahrain Institute.

It said he faces up to 15 years if convicted in that trial.

Bahrain has been the scene of frequent protests and clashes with police since security forces quelled nationwide protests that called for political reforms in 2011.