A powerful mainland task force has revealed in detail for the first time how it tracked down allegedly corrupt fugitives abroad, bringing the suspects and their assets back to China even in the absence of extradition treaties.

The revelations come as President Xi Jinping steps up his crusade against graft in the Communist Party, military and official ranks.

The Ministry of Public Security launched "Operation Fox Hunt" last year to track down fleeing officials and since then more than 500 fugitives with over 3 billion yuan (HK$3.8 billion) in assets offshore have been returned to China, according to the authorities.

In an article posted on its website on Tuesday, the party's internal graft watchdog, the Central Commission for Discipline Inspection, said the international operation was led by the Fugitive Repatriation and Asset Recovery Office, comprising officials from the CCDI, the judiciary and prosecutors, the ministries of foreign affairs, public security, state security and justice, and the People's Bank of China.

One of the operation's biggest problems is tracking down fugitives in the United States, Australia and Canada, which are attractive destinations for fleeing officials because they do not have extradition treaties with China.

The article said that in some cases China would send agents to persuade fugitives to end their exile, but in others it would provide evidence of criminal activity to host countries to repatriate them for illegal immigration directly or via a third country. The office also gave evidence so that host countries could prosecute the fugitives under local laws, the article said.

The CCDI said illicit assets were repatriated through various means, from bilateral law enforcement cooperation deals, to local civil litigation, and criminal asset seizure laws abroad or in China. Another option was to reach a settlement with the suspect or family to return any ill-gotten gains.

Meanwhile, the Financial Times reported yesterday that Politburo Standing Committee member and CCDI chief Wang Qishan was preparing to visit the US.

The London-based newspaper quoted analysts as saying that if Wang visited the US as the CCDI's head, his main mission would be to seek Washington's help on fugitives.

Western governments have long been reluctant to hand over suspects because of a lack of transparency and due process in China's judicial system.

But China has pressed the US, Australia and Canada to do more to help them bring fugitives to justice.

Before becoming the anti-graft chief, Wang played a crucial role in Sino-US relations as headed of a twice-yearly strategic and economic dialogue, which brings together the two countries' top diplomats and financial and trade officials.

The report came as the ex-wife of a fugitive former Chinese official was arrested in Washington state on allegations of immigration fraud and money laundering.

Shilan Zhao, of Washington state, was arrested on Tuesday and ordered held without bail.

US federal prosecutors contend Zhao and Jianjun Qiao falsely claimed to be married and lied about their source of money to obtain US visas through an immigrant investor programme.

An indictment unsealed on Tuesday in Los Angeles charges them with conspiracy and other crimes.