Relatives of top Chinese leaders including President Xi Jinping and former premier Wen Jiabao have used offshore tax havens to hide their wealth, according to a mammoth investigation released Wednesday.

The International Consortium of Investigative Journalists (ICIJ), citing information culled from 2.5 million leaked documents, said that Xi's brother-in-law and Wen's son and son-in-law were among those with offshore holdings.

It is the latest revelation to shine a light on the hidden wealth of family members of China's top officials, a topic considered off-limits by Communist Party leaders.

Offshore entities can be legal and there was no evidence that the politicians were aware of their relatives' actions.

The release came on the same day that China put leading activist Xu Zhiyong on trial for his part in protests calling for officials to disclose their assets.

It also came days after Wen reportedly penned a letter to a Hong Kong columnist proclaiming his "innocence" over previous claims that his family amassed huge wealth during his decade in power.

The ICIJ report cited nearly 22,000 offshore clients from mainland China and Hong Kong, including relatives of former president Hu Jintao, former premier Li Peng and late leader Deng Xiaoping, the man credited with opening up China's economy in the 1980s.

Also included were members of China's National People's Congress, heads of state-owned enterprises and some of the country's wealthiest men and women, including real estate mogul Zhang Xin; Pony Ma and Zhang Zhidong, co-founders of Chinese Internet giant Tencent; and Yang Huiyan, China's richest woman.

ICIJ said it sent letters to the government officials, wealthy individuals and others named in its report.

"Their response in most cases was to not respond, a standard practice in China," it said.

ICIJ said it collaborated with more than 50 partner organisations across the globe in sifting through the data, including The Guardian, Le Monde in France, El Pais in Spain and Hong Kong's Ming Pao newspaper.

The websites of all four were not accessible within China on Wednesday, and nor was ICIJ's.

Chinese foreign ministry spokesman Qin Gang told reporters at a regular briefing that "the logic of the article is unconvincing, which cannot but raise questions of ulterior motives".

Asked if the government planned to follow up on the report, Qin responded: "What I want to point out is, the clean will be proved clean and the dirty will be proved dirty."

The confidential files leaked to ICIJ also include the names of 16,000 clients from Taiwan.

Conflict of interest

Ninety percent of the mainland Chinese clients set up offshore entities in the British Virgin Islands, often with the help of Western firms such as UBS and PricewaterhouseCoopers, the investigation said. Seven percent were established in Samoa and three percent in other areas.

The British Virgin Islands was the destination of choice for Xi's brother-in-law Deng Jiagui, a wealthy real estate developer and investor who married Xi's older sister in 1996.

According to the report, Deng owns a 50 percent stake in BVI-based company Excellence Effort Property Development.

While such offshore trusts and companies "may not be strictly illegal", they are often linked to "conflict of interest and covert use of government power", Minxin Pei, a political science professor at Claremont McKenna College in California, told the ICIJ.

In 2012 the New York Times and Bloomberg news agency published investigations into the vast wealth said to have been amassed by family members of Wen and Xi.

Both news organisations have since had their websites blocked in China, and authorities have denounced the reports as an effort to "smear" China's leadership.

Last November the New York Times found that US bank JPMorgan hired Wen's daughter as a consultant, part of a broader strategy that the newspaper said was aimed at accumulating influence in China by employing relatives of leaders.

According to the Times report, JPMorgan paid a total of $1.8 million to Fullmark Consultants, a firm set up by Wen's daughter Wen Ruchun, who also goes by the alias "Lily Chang".

The files leaked to ICIJ purported to show Fullmark Consultants was set up in the British Virgin Islands in 2004 by her husband, who was its sole director and shareholder until 2006.

Hong Kong's Ming Pao paper on Saturday quoted Wen as writing in a letter to a columnist: "I have never been involved and would not get involved in one single deal of abusing my power for personal gain.

"I want to walk the last journey in this world well. I came to this world with bare hands and I want to leave this world clean."

It headlined its story on the ICIJ revelations about Wen but omitted any reference to Xi.