BEIJING: US officials say an envelope containing an unidentified substance was sent to the US Consulate General in the southern Chinese city of Guangzhou, disrupting operations at the diplomatic building.

Deputy spokesman Justin Higgins at the US Embassy in Beijing says the envelope was opened Monday in an office where visa interviews are conducted. Higgins says interviews scheduled yesterday have been canceled while an investigation is carried out.

He says no one was injured and that precautionary measures have been taken for employees present when the substance was discovered. He would not describe the substance, which he says has yet to be identified.

The website of Guangzhou’s Yangcheng Evening News reported that the substance was a powder and that it had sprinkled onto an employee’s clothes.

Meanwhile, Chinese authorities have shut down or frozen the microblog accounts of several prominent liberal intellectuals and harassed rights lawyers lobbying against unofficial “black jails,” underlining the determination of the country’s new leadership to control dissent even as it vows to root out corruption.

The moves over the last few days occurred around the time officials announced that a senior official was being investigated for graft, months after a prominent journalist accused him of wrongdoing. The probe against Liu Tienan, deputy chairman of China’s economic planning agency, was heralded by the Chinese press as proof that the battle against corruption is best fought when authorities allow public participation.

“The authorities and the people combined their strengths in this case, and it is an encouragement to the public’s power in fighting corruption,” said a state-run daily, the Beijing News, in a commentary.

The government issued a defense of its human rights policies in a report Tuesday, outlining progress made in improving health, welfare and other living standards — a key measure the government uses in its definition of rights. The report said China takes measures to ensure the “citizens’ right to know and right to be heard.” But the authoritarian government also has shown an unwavering intent to clamp down on anyone who seeks to publicly pressure it into social or political change. The message appears to be that if any reform is on the agenda, the Communist Party will push it through on its own terms.

“The controls are tighter than ever,” said Li Cheng, an expert on China’s elite politics at the Washington-based Brookings Institution. “The challenges are greater, so the suppression is escalating.”