An American human rights envoy said Thursday that China provided no useful information when probed about specific cases of individuals who have been detained or who disappeared in a major crackdown on dissent in recent months.

Hundreds of lawyers, activists, and other intellectuals have been questioned, detained, confined to their homes or have simply disappeared, apparently to squelch any chances of the kind of popular uprisings roiling the Middle East and North Africa. The clampdown on dissent is the broadest and harshest in years by China's Communist government.

Michael Posner, U.S. assistant secretary of state for democracy, human rights and labor, said that his delegation received no satisfactory answers to its questions about Teng Biao, a law professor who disappeared in February, and the artist Ai Weiwei, who was apparently detained by authorities April 3 but has yet to be formally charged.

"We need to and will continue to raise these issues in a range of forums," Posner said. "The most senior government officials in the United States are deeply concerned about the deterioration of human rights in China over the last several months."