As Beijing's crackdown on corruption continues, top figures in Macau's casino junket sector are coming under scrutiny ahead of a visit to the city later this week by President Xi Jinping. After a week in which the starkest warnings yet were delivered by senior mainland figures that Macau must move away from its over-reliance on gaming revenues, it has emerged that Hong Kong police have frozen the assets of key casino junket operator Cheung Chi-tai. Junkets are the businesses that provide VIP customers to casinos and bring in the bulk of their revenue. Cheung - who was alleged to be a triad member during the money-laundering trial of Carson Yeung Ka-sing, the former owner of soccer club Birmingham City - is wanted for questioning by detectives in Hong Kong investigating offences under the Organised and Serious Crimes Ordinance. Cheung has not been charged and could not be contacted for comment. The investigation is one of a series into gaming and property deals under Xi's "tigers and flies'' anti-corruption drive, which has stretched into the heart of the Communist Party and led to an international effort to snare and bring back graft suspects from overseas. Xi's visit to Macau will mark the 15th anniversary of the former Portuguese enclave's return to Chinese sovereignty. Prior to becoming president in 2012, Xi headed China's advisory team on Hong Kong and Macau affairs for four years. His corruption crackdown - coupled with an economic slowdown on the mainland - has hit Macau's casino revenues hard. In October, casino revenue fell 23.2 per cent to 28 billion patacas, the largest monthly drop since authorities began releasing statistics to the public in 2005. More than two-thirds of the Macau government's revenue comes from gaming taxes. Junkets are an obvious target of graft-busters because of long-standing accusations they provide underground cash transfers from the mainland to Macau and Hong Kong, helping mainlanders avoid stringent capital controls. The most prominent such case yet led to the sentencing in 2010 of China's then-richest man, Huang Guangyu, to 14 years in prison for bribery and insider trading. Associates of Cheung in the junket industry assisted Huang in transferring money to Hong Kong, the court heard at the time. One long-time observer of the Macau gambling industry said that junket operators and casinos had made an effort to cut ties with people targeted in criminal investigations amid increasing scrutiny from Beijing. "All ties would have been cut earlier this year," the person said. Hong Kong Police declined to comment on the investigation into Cheung, and Macau's Judicial Police said - as a matter of policy - they would not reply to requests for comments from media based outside the gambling hub. Cheung's name has also appeared in court documents in Las Vegas in an ongoing illegal bookmaking case there with strong links to Macau and Hong Kong. A photocopy of Cheung's passport was found by the FBI during a raid in July on three luxury Caesars Palace villas, believed by federal authorities to be the headquarters for an illegal multibillion-dollar World Cup betting scheme. The passport copy was labelled under the file name "HK Boss". The raid led to the arrest of Paul Phua Wei-seng, a Malaysian citizen who US federal agents said was a "high-ranking member" of the 14K triad. Prior to his arrest in Las Vegas, Phua had been detained in Macau in June, in what was the city's largest crackdown on illegal gambling. Police in Macau claimed the gambling operation took HK$5 billion in wagers. His lawyers contest the charges and say police have shown insufficient evidence to support the triad allegation. The trial's other defendants - who include three Hongkongers and Malaysian junket operator Richard Yong Seng-chen - this week pleaded guilty to misdemeanours and were sentenced to unsupervised probation and fines. Macau casinos cut their ties with junkets operated by Phua and Yong earlier this year.