GENEVA (Reuters) - Switzerland opened a war-crimes investigation into Rifaat al-Assad, an uncle of Syrian leader Bashar al-Assad, almost four years ago, but it has stalled despite “compelling evidence”, an activist group and lawyers said on Monday.

The Geneva-based group TRIAL International said that it had brought the complaint against Rifaat al-Assad for alleged massacres in Tadmor in 1980 and Hama in 1982, and was joined by Swiss lawyers for six Syrian plaintiffs seeking justice.

“He was in command of the Defense Brigades – the country’s elite commando troops – in the 1980s,” the group said in a statement. “Under his lead, the Brigades are suspected of participating in the massacre of several thousand people in Tadmor and Hama.”

Following TRIAL’s complaint, an investigation of Assad - the uncle of Bashar al-Assad and the younger brother of his father, Hafez Assad, Syria’s former president - began in 2013, the group said. But the criminal inquiry “seems to be at a standstill”, the plaintiffs’ lawyers, led by Damien Chervaz, said in a statement.

The Swiss attorney general confirmed that an investigation into war crimes was opened in December 2013 against a Syrian national who was the alleged commander of a military unit in the 1980s, but declined to confirm his identity.

Its investigations were time-consuming because of the “complex” nature of incidents that occurred long ago in another country, the attorney general’s office said in a statement. A spokeswoman said it had no further comment on any criticisms.

Marc Hassberger, Geneva lawyer for Rifaat, told Radio Television Suisse (RTS) on Monday night that his client rejected the accusations.

He added: “Obviously one must be wary of outrageous simplifications regarding a procedure that concerns facts which may have happened several decades ago and also thousands of kilometers away from here.”

A former Syrian vice president who was sent into exile in the 1980s, Assad lives in France, where he was put under investigation for tax fraud and money-laundering last year. In April, Spain confiscated property he owned, as part of a investigation into alleged money-laundering.

Khaled Al-Khani, a Syrian artist who said that his father was tortured and killed in the crackdown on Hama in 1982, when he was a child, is a plaintiff in the Swiss case. “If justice can be rendered to victims of Srebrenica or Rwanda, one must do the same for those whom we have lost,” he told Swiss television.