A top official linked to one of China's most powerful politicians has been placed on leave for stress, the government said Wednesday, amid unconfirmed speculation that he had tried to flee the country.

Wang Lijun, vice mayor and former police chief of Chongqing, is famed as one of China's top graft-busters after leading a crackdown that led to scores of senior officials being jailed in the southwestern city of 30 million people.

He has close links with Chongqing's Communist Party secretary Bo Xilai, who is the son of a Chinese revolutionary and is widely expected to be promoted to a top party post in a 10-yearly leadership transition that begins this year.

Wang was dismissed as Chongqing police chief last week, and on Wednesday, Chinese websites buzzed with rumours he had sought asylum at the US consulate in the southwestern city of Chengdu.

The Chongqing government refused to comment when contacted by AFP, but a posting on an official microblog said Wang was on sick leave -- a term often used as a euphemism for a political purge in China's one-party system.

"Due to long-term overwork, a high-level of mental stress and physical exhaustion Vice Mayor Wang Lijun is currently receiving vacation-style treatment following approval," said the provincial government.

As Bo's right-hand man, Wang, 53, an ethnic Mongolian, gained national fame while toppling former city deputy police chief Wen Qiang in a massive crackdown. Wen was executed in 2010.

The crackdown led to the arrest and detention of dozens of triad bosses and hundreds of their protectors in government and the police.

The US embassy in Beijing refused to comment on the speculation of an asylum request, which comes at a sensitive time with China's vice president and leader-in-waiting Xi Jinping due to make an official visit to Washington next week.

"I am not in a position to comment on issues regarding reported requests for asylum," embassy spokesman Richard Buangan told AFP.

"We cannot comment on issues regarding the security of diplomatic facilities," he added when asked about reports of an increased police presence at the Chengdu consulate.

Calls to Chengdu police went unanswered Wednesday.