Bahrain's Military High Court on Monday sentenced six Shiites to death, 15 years in prison and revoked their nationality on charges of attempting to assassinate the commander-in-chief of the country's defense force, who oversees the appointment of military judges.

The state-run Bahrain News Agency said the case involved 18 defendants, eight of whom were tried in absentia. The group was charged with a number of crimes, including forming a terrorist cell.

Seven of the defendants were sentenced to seven years in jail and stripped of their Bahraini nationality. Five were acquitted. The state news agency said those found guilty have the right to appeal the verdict with the military's high court of appeals.

The Bahrain Forum for Human Rights criticized the verdicts, saying the military's judiciary is prosecuting civilians for simply exercising their right to freedom of expression and peaceful assembly. It alleged that the defendants were subjected torture and solitary confinement.

Bahrain, a tiny island-state off the coast of Saudi Arabia, is majority Shiite, but ruled by a Sunni monarchy. The country, which hosts the U.S. Navy's 5th Fleet, was roiled in Shiite-led protests in 2011, but the government cracked down on protesters with help from Emirati and Saudi troops.

Since the crackdown, some Shiite militant groups in Bahrain have launched deadly attacks on police and security forces. Bahrain's government accuses Shiite-led Iran of backing militant groups in their attacks — a charge Tehran denies.

Since 2011, military courts have tried hundreds of defendants. A government-appointed investigation after the protests criticized the use of the courts, saying they were employed "to punish those in the opposition" and raised "a number of concerns about their conformity with international human rights law."