Saudi Arabia on Thursday provisionally released two Saudi-Americans after nearly two years of detention, campaigners and sources close to their families said, as the kingdom's rights record comes under renewed US scrutiny.

Salah al-Haider, the son of a leading women's rights activist and Bader al-Ibrahim, a writer and doctor, were detained in April 2019 and charged with terrorism-related crimes.

Their release was reported by campaign groups Prisoners of Conscience and the Washington-based Freedom Initiative as well as two sources close to both their families who requested anonymity.

"This is welcomed progress, even though it is long overdue," Freedom Initiative's Bethany al-Haidari said in a statement.

"Bader al-Ibrahim and Salah al-Haider should have never been jailed in the first place and their release should certainly not be on a 'temporary' basis."

They were released pending trial in the kingdom's Specialized Criminal Court (SCC) and their next court hearing is set for March 8, Freedom Initiative said.

Campaigners accuse the kingdom of using the SCC, or anti-terrorism court, to silence critical voices under the cover of fighting terrorism.

"We welcome and are closely monitoring the conditional release of U.S. citizens Badr Al-Ibrahim and Salah Al-Haider," a State Department spokesperson told AFP Thursday. 

Saudi authorities have not officially commented on the pair's detention, charges or provisional release, which comes after Saudi Arabia accelerated political trials following US President Joe Biden's election victory late last year.

During his election campaign, Biden had pledged to make the kingdom a "pariah" over its human rights failings after it largely got a free pass under President Donald Trump.

Last month, a Saudi appeals court upheld the conviction of prominent Saudi-American doctor Walid Fitaihi on charges that included "disobedience" against the kingdom's rulers, a source close to his family told AFP.

But the Harvard-educated doctor will effectively serve no jail time after the court reduced his six-year jail term handed down in December by half, and suspended a part of the remaining sentence, the source said.

Fitaihi, who still faces a travel ban and an assets freeze, has already served nearly two years in pre-trial detention.

The State Department spokesperson also welcomed the reduction of Fitaihi's sentence. 

"We remain in close contact with family members and we will continue to advocate for a swift resolution" of all three cases, they added.

Under Crown Prince Mohammed bin Salman, the kingdom's de facto ruler, Saudi Arabia has detained activists, clerics and royal family members in a sweeping crackdown on dissent over the last three years.

The detentions have cast a spotlight on the human rights record of the kingdom, an absolute monarchy which has also faced intense criticism over the 2018 murder of journalist Jamal Khashoggi in its Istanbul consulate.