The mystery over the reported abduction from Hong Kong of a Chinese billionaire deepened Wednesday after a newspaper advert appeared in his name pledging loyalty to China, in a case that has heightened fears over Beijing's meddling.

The whereabouts of financier Xiao Jianhua -- one of China's richest men -- are unclear after reports in overseas Chinese-language media that he was taken from Hong Kong by mainland security agents last week.

The reports suggested Xiao's disappearance was part of China's ongoing anti-corruption campaign, which some critics believe has been used to target President Xi Jinping's political opponents.

Xiao had been staying at Hong Kong's Four Seasons hotel and was looked after by female bodyguards, local media reported.

A front-page advert in Hong Kong newspaper Ming Pao, attributed to Xiao, said he had "always loved the (ruling Communist) party and the country" and would soon meet with media.

"I personally believe the Chinese government is civilised and has rule of law," the advert read.

"I have not been kidnapped."

Xiao, who said in the statement he was a Canadian citizen, insisted he was being treated for an illness overseas.

The founder of Beijing-based Tomorrow Group, Xiao was previously reported to have denied allegations he fled to Hong Kong in 2014 to escape the corruption crackdown.

He is said to have acted as a broker for the Chinese leadership, including for Xi's family.

But overseas Chinese-language news site Bowen Press said Xiao could also have been connected to an "anti-Xi coalition".

It is illegal for mainland agents to operate in semi-autonomous Hong Kong, but the disappearance of five booksellers known for publishing salacious titles about Beijing's leadership in 2015 prompted widespread criticism China had overstepped that line.

One of the men, Lee Bo, vanished from Hong Kong, triggering international condemnation and local protests.

Lee always insisted he had gone over the border voluntarily.

Hong Kong's security bureau said the government "will not allow non-Hong Kong law enforcement officers to take law enforcement actions in Hong Kong".

But James To of the Democratic Party said there was a "credible suspicion" Hong Kong's "one country, two systems" deal had been breached.

"After the Lee Bo fiasco people are very concerned about whether Hong Kong residents or people lawfully staying in Hong Kong will be protected," To told AFP.

- 'Credible suspicion' -

The South China Morning Post reported Xiao was currently in mainland China, not receiving medical treatment, and was only in contact with his family.

Xiao had wanted to move some of his businesses to Japan after feeling unsafe in Hong Kong, Initium news site said.

His wife was now in Japan after reporting his case to Hong Kong police then withdrawing it, the report said.

According to the Financial Times Xiao was led away by Chinese public security agents from an apartment at the Four Seasons.

The hotel said there was an "active police investigation" when asked about the case.

Hong Kong police said they had received a request for assistance over a "mainland citizen" Saturday, but a family member had later retracted it.

They said the person it referred to had crossed a border control point between Hong Kong and China on Friday.

The Canadian consulate said it was aware of the reports and its officials were "in contact" with authorities.

Xiao said in the Ming Pao statement he was a permanent resident of Hong Kong, and held a diplomatic passport.

In 2015, he was made "ambassador-at-large" for Antigua and Barbuda, according to news site Caribbean360, which pictured Xiao at the time with the country's prime minister Gaston Browne.

Hong Kong-based analyst Willy Lam said Xiao may have been targeted because he knew "potentially embarrassing details" about financial actions involving major Chinese political clans.

China's anti-corruption drive was launched after Xi took power in 2012 and has brought down government officials and corporate executives.

Billionaire Guo Guangchang, chairman of one of the country's biggest private-sector conglomerates Fosun, vanished from public view in 2015 in connection with an investigation by authorities, and then re-emerged.

In 2016 the chairman of one of China's most prominent fashion firms also disappeared, returning to work a week later amid speculation he had been caught up in the anti-corruption campaign.