A senior U.S. envoy is calling on Bahrain to prosecute those responsible for widespread crackdowns during the early stages of an anti-government uprising in the Gulf kingdom last year, The Associated Press reported.

Assistant Secretary of State Michael Posner also urged Bahrain's leadership to drop charges against activists involved in "non-violent political expression" and voiced concern about recent measures, including revoking citizenship for 31 activists.

Posner's statement suggests stronger demands by Washington toward strategic ally Bahrain, which is home to the U.S. Navy's 5th Fleet. There was no immediate comment from Bahraini officials on Tuesday.

Posner says Bahrain's Sunni rulers need to take steps to open credible talks with the country's Shiite opposition groups. He also condemned violence by protesters.

More than 55 people have been killed in Bahrain's unrest since February 2011.