A former Chinese police chief whose visit to a US consulate sparked rumours he was trying to defect flew to Beijing with a top state security officer after meeting American officials in southwest China.

Wang Lijun, who has close links to a high-profile contender to join China's top decision-making body, flew first class from Chengdu to Beijing on February 8, according to a travel website authorised to show passenger flight details.

Qiu Jin, who is the deputy head of China's State Security Ministry, also had a first class seat on the same flight. The website Travelsky, which is backed by China's aviation regulator, shows the tickets were used.

It is not clear if the two men were travelling together, but the fact they were on the same flight will fuel speculation about Wang whose exact whereabouts are unknown.

Wang, 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.

The operation brought Wang into the spotlight and boosted the political prospects of his controversial boss, Chongqing Communist Party secretary Bo Xilai, who rewarded Wang by making him his deputy.

Wang, 52, an ethnic Mongolian and martial arts expert, was dismissed as 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.

Chongqing authorities said Wang was on leave, receiving "vacation-style treatment" for stress and over-work.

Sick leave has frequently been used in the past as a euphemism for political purges under China's one-party system.

The US embassy in Beijing confirmed Wang's visit to the consulate this week but declined to comment on the rumours he had sought asylum, saying only that he had gone there for a meeting and left "of his own volition".

China's official Xinhua news agency said authorities were "investigating the incident".

Analysts said the confirmation of the visit would stoke rumours surrounding Wang and Bo, and said it could hamper Bo's chances of promotion to the nine-member Politburo Standing Committee, China's top decision-making body.

"It looks like this investigation of Wang Lijun is part of an offensive against Bo -- that is the usual pattern in China," Jean-Pierre Cabestan, a China scholar at Hong Kong Baptist University, told AFP.

Bo 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.

Known as a "princeling" due to his father's revolutionary legacy, Bo has encountered opposition from those who are against nepotism and hereditary rights in China's political system.

Bo's crackdown on corruption and organised crime in Chongqing was widely popular, although responses to his campaign to instil "red" or communist-style patriotism in the municipality were mixed.

"Bo is seen as a Machiavellian figure who is willing to risk anything to achieve his goals," Willy Lam, a leading China expert at the Chinese University of Hong Kong, told AFP this week.

"His high-profile campaign to sing red songs and crack down on triads are regarded as cynical ploys to boost his own political standing."