Bahrain's leading Shi'ite cleric on Friday slammed the Gulf kingdom's leaders for a continuing crackdown on dissent and called the rulers' reform efforts “cosmetic.”

The Friday sermon outside the capital, Manama, by Sheik Isa Qassim is the latest rumbling from the Shi'ite majority against Bahrain's minority Sunni group which runs the country. Shi'ites make up about 70 percent of the population but have suffered in a widespread government purge of demonstrations and dissent led by Shi'ites.

The denouncement comes after a top U.S. rights envoy this week expressed concern about alleged abuses against people detained in Bahrain's crackdown.

Assistant Secretary of State Michael Posner cited reports that some detainees have endured physical abuse during interrogations.

Posner also voiced optimism about proposed talks next month between the country's Sunni rulers and opposition groups. Shi'ite leaders, however, want to see government concessions before agreeing to begin talks.

U.S. President Barack Obama has criticized Bahrain's crackdown on anti-government protesters that left 24 people dead in March. The Sunni-ruled nation has also arrested hundreds of mostly Shi'ite protesters and put dozens on trial in special courts.

Bahrain has insisted it is open to dialogue and has denied reports of human rights abuses.

Rights groups say hundreds of people are on trial in what they call a revenge move by the kingdom's Sunni rulers who put down anti-government protests led by the country's Shi'ite majority.

Bahrain is a key U.S. ally in the Persian Gulf and home to the U.S. Navy's 5th Fleet.