A Saudi court sentenced human rights activist Issa al-Nukheifi to six years in prison on Wednesday for tweets critical of the government, a rights group said, confirming a Saudi newspaper report.

On Tuesday, Essam Koshak, another activist, was handed a four-year sentence, Al Jazeera Arabic reported, citing online campaigners.

Saudi newspaper Okaz said Nukheifi, whom it did not identify by name, was accused of criticising Saudi Arabia's intervention in Yemen, insulting the government and objecting to its decisions, criminal proceedings and security procedures.

ALQST, a London-based Saudi rights group, identified the defendant as Issa al-Nukheifi, an activist who was detained in December 2016. It condemned the conviction and sentencing.

Amnesty International called for his immediate release. "Instead of engaging with activists on reforms, the authorities are going after them one by one, with almost all of the country’s human rights activists now silenced or imprisoned," said Dana Ahmed, the group's campaigner on Saudi Arabia.

Okaz reported that Nukheifi's was also sentenced for re-tweeting anti-government accounts and simultaneously receiving money transfers from a prisoner still serving a sentence.

Mohammed bin Salman has no real interest in improving his country’s record on free speech and the rule of law. - Sarah Leah Whitson, HRW

Nukheifi was sentenced to six years of prison and, following his release, a six-year ban on social media as well as a six-year travel ban outside of the country.

He had been previously arrested, tried and sentenced to prison for similar offences, an Amnesty report said. He was detained in 2012 and released in April 2016, only to be arrested eight months later.

On Tuesday, ALQST published a letter written by Nukheifi from custody to Crown Prince Mohammed bin Salman.

"I was very happy listening to your speeches and interviews where you call for freedom of expression and respecting human rights, which is what we call for and work with you to achieve," he wrote. "I am writing to you from inside my imprisonment, having been detained for asking for these [steps]."

Social media activists also reported that Koshak was sentenced to four years on Tuesday.

🔴Human rights activist Essam Koshak ( #عصام_كوشك ) has just been sentenced in the #Saudi Terrorism Court to 4 years in prison followed by a 4-year travel ban, for "inciting public opinion" !! pic.twitter.com/A0U6L3WNxH — Prisoners of Conscience (@m3takl_en) February 27, 2018

"Koshak is an independent activist and computer engineer who has used social media sites such as Twitter to push for human rights in Saudi Arabia, including highlighting the repression of peaceful activists and dissidents and advocating their release," Human Rights Watch said after the activist's arrest in January 2017.

In September, Saudi authorities arrested about 30 clerics, intellectuals and activists.

HRW Middle East director Sarah Leah Whitson said at the time that the arrests were a sign that "Mohammed bin Salman has no real interest in improving his country’s record on free speech and the rule of law."