A special Bahrain security court has sentenced four demonstrators to a year in jail for involvement in anti-government protests, a human rights group said Wednesday.

Three of the protesters were convicted a day earlier on charges that included taking part in illegal protests, according to the Bahrain Youth Society for Human Rights. It said the fourth was found guilty of possessing pamphlets calling for the overthrow of the country's ruling system.

The reported convictions were part of a series of closed-door trials in a special court set up in March during a crackdown on the Shiite-led protests. In other cases, a military prosecutor has presented evidence before the court, which is presided over by military and civilian judges, the group said.

The rights group has also raised concerns about a 15-year-old it says is being tried before the court. The teenager, Mohammed Ebrahim Khatem, faces charges of gathering in a group of more than five people without authorization and rioting, according to the group.

A government spokeswoman did not respond to a request for comment on the cases.

The court is separately trying 21 opposition leaders and political activists, most of them Shiites, accused of plotting against the state. It has sentenced four people to death for the alleged killing of two police officers during the unrest, but an appeals arm of the court reduced two of those sentences to life behind bars.

At least 30 people have been killed since anti-government protests began in mid-February by the majority Shiites, who are seeking greater rights and political freedoms from Bahrain's Sunni rulers.

The country is an important U.S. ally in the Persian Gulf and hosts the U.S. Navy's 5th Fleet. President Barack Obama last week called on Bahrain's leaders to create conditions for dialogue with the opposition, adding that "you can't have a real dialogue when parts of the peaceful opposition are in jail."

Also, Bahrain's interior minister has ordered a "speedy investigation" into allegations that a female news agency correspondent was mistreated while in police custody, according to the official Bahrain News Agency.

Bahrain's Information Affairs Authority confirmed the journalist was Nazeeha Saeed, who works for French television channel France 24 and Radio Monte Carlo. France 24 has said she was detained for questioning and then released earlier this week.