Rifaat al-Assad, the uncle of Syrian President Bashar al-Assad, is being investigated for the suspected slaughters of thousands of civilians during the 1980s by military units under his command.

Rifaat is additionally involved in French and Spanish corruption probes.

Assad's visits to Switzerland allowed Swiss authorities to arrest him in relation to major international crimes.

The case is currently at a stalemate.

The uncle of Syrian President Bashar al-Assad is being investigated by Switzerland for alleged war crimes in the 1980s, prosecutors said on Monday, as a rights group demanded progress in the stalled four-year case.

A previously undisclosed investigation is targeting Rifaat al-Assad, the older brother of the late Syrian dictator Hafez al-Assad.

Rifaat - who is separately implicated in French and Spanish corruption probes - was forced into exile in 1984 after a botched attempt to overthrow his brother Hafez from power.

But Swiss authorities are investigating alleged offenses committed when Rifaat al-Assad commanded military units suspected of slaughtering thousands of civilians during the 1982 Hama massacre.

In a statement, the Swiss attorney general's office (OAG) said that following a criminal complaint it had opened a "war crimes" case against a Syrian national in December 2013, in connection alleged offenses committed in Syria in the 1980s.

The complaint was filed by TRIAL International, a rights group that works with victims and pushes Switzerland to prosecute alleged global criminals.

TRIAL said that much of the evidence it had compiled against Assad relates to his role in suppressing a 1982 rebellion in the Syrian city of Hama, where as many as 10,000 people were killed in the brutal military campaign.

TRIAL said that Assad's occasional visits to Switzerland gave authorities the opportunity to arrest him, in accordance with the Swiss principle of universal jurisdiction for major international crimes.

"The case is at a stalemate, despite the significant amount of evidence at hand," TRIAL said in a statement.

"Executions, shelling, torture, rape, looting: in spite of the gravity of the crimes, nobody has ever been prosecuted."

Rifaat Al-Assad, in exile since 1984, has "traveled freely for years and invested a vast personal fortune in Europe", TRIAL added in a press release.

Trial spokeswoman Chloe Bitton told AFP that the decision to go public four years after the probe was opened was made by lawyers representing Assad's victims, who were frustrated with the lack of progress, while his nephew continues the mass slaughter of Syrians in his attempt to hold power.

The OAG said the "very complex" nature of the investigation meant the probe would take time.

The Syrian conflict began when the Baath regime, in power since 1963 and led by Assad, responded with military force to peaceful protests demanding democratic reforms during the Arab Spring wave of uprisings, triggering an armed rebellion fuelled by mass defections from the Syrian army.

According to independent monitors, hundreds of thousands of civilians have been killed in the war, mostly by the regime and its powerful allies, and millions have been displaced both inside and outside of Syria.

The brutal tactics pursued mainly by the regime, which have included the use of chemical weapons, sieges, mass executions and torture against civilians have led to war crimes investigations.

This article has been adapted from its original source.