Reuters: The United States expressed concern on Tuesday about the treatment of opposition figures in Bahrain, following the arrest of a prominent Sunni Muslim leader for plotting to overthrow the monarchy three weeks after he was freed from jail.

Bahrain's Interior Ministry said on Sunday that Ibrahim Sharif, former head of the secular National Democratic Action Society, or Waad, was detained for incitement to overthrow the government and publicizing "hatred of the regime" in a speech on July 10.

Sharif had been freed by royal pardon on June 19 after serving more than four years in prison for his role in an uprising demanding political reforms in the Gulf Arab island monarchy.

The U.S. State Department said the arrest of Sharif, the detention and prosecution of opposition figure Majeed Milad, and reports about the re-opening of a case against Ali Salman, secretary general of Al Wefaq Political Society, "raise serious concerns about restrictions on freedom of expression in Bahrain."

In the statement, State Department spokesman John Kirby said the United States welcomed initial reports that rights activist Nabeel Rajab had been released from jail, but continued to encourage the government to respect freedom of expression.

Bahrain, which hosts the U.S. Fifth Fleet, has experienced sporadic turmoil since mass protests in 2011 led by majority Shi'ites demanding reforms and a bigger role in government - an uprising put down with military help from Saudi Arabia.

Bahrain's official BNA news agency reported on Monday that Rajab, one of the Arab world's best-known democracy activists, had been freed from a six-month jail term handed down in May for insulting the authorities. It cited health reasons for the release.

Bahrain says the opposition has a sectarian agenda and is backed by Shi'ite power Iran, a charge Shi'ite groups deny and that Waad - a secular leftist party run by both Sunnis and Shi'ites - has said is contradicted by its own secular approach.