UK-based Saudi rights organisation ALQST said Hatoon al-Fassi, Amal al-Harbi, Maysaa al-Manea, and Abeer Namankani were all released, with reports a fifth had also been let out.

They are among 11 women held for about a year on charges related to the country's cyber-crimes law.

Saudi officials are yet to comment.

The terms of their release and when they will return to jail are unclear.

The government in Riyadh has come under international pressure to free the women who were arrested last year in May shortly before authorities lifted a ban on women driving.

Eman al-Nafjan, Aziza al-Yousef and Roqaya al-Mohareb were all freed in March.

Why were they arrested?

All 11 women were put on trial in March. When they were detained in 2018, the public prosecutor's office said they were suspected of harming national interests.

They have been accused of contact with foreign journalists and human rights organisations.

Some of the women in court appearances say they have been electrocuted, flogged and sexually harassed in prison, allegations Saudi authorities have denied.

In March 36 states at the UN Human Rights Council jointly condemned Saudi Arabia for the women's detentions, the first collective rebuke of the Gulf kingdom since the council was set up in 2006.

 Eman al-Nafjan and Aziza al-Yousef were freed in March, but Loujain al-Hathloul remains in prison

While the release of the three activists in March ignited hopes for change, in April the kingdom detained a number of other activists - including two dual US-Saudi citizens and a pregnant women.

The group are said to be bloggers and writers who have discussed reform.

Scrutiny of Saudi Arabia's human rights record has intensified since the murder of journalist Jamal Khashoggi at the Saudi consulate in Istanbul last October.

Turkish investigators and others have pointed the finger at Crown Prince Mohammed bin Salman, seen as the real power behind the throne, alleging he orchestrated the murder.

But the Saudi authorities deny he was involved and blame a "rogue" operation. Eleven people went on trial in January.

The arrests of activists and writers are seen as an attempt to shut down criticism of the crown prince, who has himself enacted some reforms.