A prominent human rights activist has been arrested by Bahraini authorities, a statement said Saturday, in a move that could further escalate the nearly 15-month-old unrest between opposition groups and the rulers of the Gulf kingdom.

A statement from Bahrain’s Interior Ministry said public prosecutors ordered the arrest of Nabeel Rajab, president of the Bahrain Center for Human Rights. The group has been active in calling attention to alleged abuses by Bahraini security forces and crackdowns such as widespread arrests and workplace purges. Rajab also was affiliated with international groups such as Human Rights Watch.

The statement gave no other immediate details. Rajab had been previously detained by Bahraini forces last year during the beginning of the Shiite-led uprising in the strategic kingdom, which is home to the U.S. Navy’s 5th Fleet.

Earlier, Bahraini officials say four policemen were injured in the latest small-scale blast targeting security forces.

More than 50 people have died in the unrest between the forces of the ruling Sunni monarchy and the country’s majority Shiites, which seek a greater voice in the nation’s governmental affairs.

The Interior Ministry statement says the blast was caused by a remotely detonated device in a mostly Shiite area about 10 miles (15 kilometers) west of the capital Manama.