China's ruling Communist Party said Friday it has put the deputy chief of the country's top intelligence agency under investigation, the latest high-ranking figure probed in a much-publicised corruption crackdown.

Ma Jian, a deputy head of China's ministry of state security, is suspected of "serious disciplinary violation" -- generally a euphemism for graft -- the party's internal watchdog said on its website.

The shady ministry of state security is said to be responsible for intelligence gathering overseas and surveillance against Chinese dissidents. It is a vast organisation but does not have a website or public address.

Separately, the former mayor of the eastern city of Nanjing, Ji Jianye, stood trial Friday for bribery.

The news of Ma's investigation follows probes into other figures in China's security apparatus, most notably Zhou Yongkang who was responsible for the security ministry and Ma's ultimate boss as a member of the party's all-powerful Politburo Standing Committee before his retirement in 2012.

Dozens of Zhou's associates and family members, including many from the police and security services, have been detained in the past year, according to Chinese reports.

State media reported Thursday that Zhou had formed a clique with Bo Xilai, a former rising star in the party who fell victim to a murder and graft investigation and was jailed in 2013.

China's President Xi Jinping has vowed to target both high-level "tigers" as well as low-level "flies" in a campaign against endemic graft that he says is a threat to the future of the party.

But critics say China has failed to implement institutional safeguards against graft, such as public asset disclosure, an independent judiciary, and free media, leaving anti-corruption campaigns subject to the influence of politics.

A party investigation usually precedes a criminal prosecution, followed by a trial and possibly a jail sentence.

Former Nanjing mayor Ji, who was expelled from the ruling party last January, allegedly accepted 11.3 million yuan ($1.9 million) from late 1999 to 2012, the Yantai Intermediate People's Court said on its microblog.

The politician had earned the nickname "Bulldozer Ji" for promoting construction in the city -- but state media linked his downfall to construction project awards to a company with which he had close ties.

- Police meeting -

The ministry of state security is often described as China's equivalent of the Soviet Union's much-feared KGB.

Li Fengzhi, a former operative in the ministry who defected to the US, told reporters in 2009 that he had grown "furious" that his job entailed spying on dissidents, spiritual groups and aggrieved poor people.

Very little official information about Ma has been made public, but respected financial magazine Caixin said he had worked at the ministry for several decades after attending university in the 1980s.

It cited an unnamed source as saying Ma was connected to "disputes" at Founder Group, a technology conglomerate affiliated with the elite Peking University.

Police this month reportedly detained the group's CEO and several other executives after they were accused by a business rival of insider trading and misappropriating company assets worth several billion yuan.

The most recent official report to mention Ma says that he attended "activities related" to a December meeting in Islamabad between China's top police official Guo Shengkun and Pakistani Prime Minister Nawaz Sharif.

Hong Kong's South China Morning Post this week cited sources as saying that Ma is "closely linked" to Ling Jihua, the previous chief of staff to former president Hu Jintao.

It added that Ma may have been held as part of an inquiry into Ling, who was detained last month following rumours that he had attempted to cover up the lurid 2012 death of his son in a Ferrari crash.