The U.S. State Department has expressed deep concern over the detention of Bahraini opposition leader Sheikh Ali Salman, warning that the arrest could lead to more tensions in the island kingdom.

The U.S. State Department has expressed deep concern over the detention of Bahraini opposition leader Sheikh Ali Salman, warning that the arrest could lead to more tensions in the island kingdom.

Sheikh Ali Salman, head of the al-Wefaq Islamic Society, was arrested on Sunday after leading a protest rally against last month's elections which his party boycotted.

He was remanded in custody for a further week on Tuesday.

"Opposition parties that peacefully voice criticism of the government play a vital role in inclusive, pluralistic states and societies," the U.S. State Department said in a statement on Wednesday.

"We are concerned that this action against a senior leader of the opposition will only inflame tensions," it said.

The United Nations human rights office in Geneva earlier said it was concerned that a potentially lengthy prison sentence could be handed to Sheikh Salman.

Sheikh Salman's lawyers have said he has been accused of serious charges, including calling for the overthrow of the government.

Bahrain, whose government has close relations with the United States and Britain, has been gripped by tension since 2011 protests demanding reforms and a bigger role in running the country.

Separately, a Bahraini court on Wednesday sentenced prominent human rights activist Mohammad al-Maskati to six months in jail on charges of taking part in an illegal gathering in 2012, his lawyer said.

"Today's verdict pushes Bahrain another step away from the healing it needs," Human Rights First said in a statement. "Targeting civil society leaders like al Maskati polarizes and divides the country further."