Nearly 1.2 million people have been punished in China's ongoing anti-graft campaign, a senior official has said, promising that the government will double-down on the drive that some say is a political purge.

As part of the campaign, nearly 2,600 fugitives have been extradited or repatriated, and 8.6 billion yuan ($1.2 billion) worth of assets have been recovered, the Central Commission for Discipline Inspection (CCDI), said in a statement published on the organisation's web site Monday.

China has extradition treaties with 48 countries in total, including Spain, Italy, and France, which returned a fugitive to China last September.

Speaking at a press conference Monday, Luo Dongchuan, a senior CCDI official pledged that the campaign would continue, saying that the government should "keep the crushing momentum against corruption," according to a press briefing transcript posted on the group's website.

But in 2016 the number of corruption cases reported to the watchdog decreased for the first time since 2012, another official, Xiao Pei, said, without giving a specific figure.

He said of the 2016 cases, 57,000 were self-confessions.

Around 410,000 officials, 76 of whom ranked at the ministerial level or above, were punished in 2016, the state-run China Daily reported Monday.

The CCDI statement followed an announcement that Beijing would be tightening the reins on anti-corruption investigators after more than 7,900 across the country, including 17 from the CCDI itself, were punished for their own wrongdoing.

Government corruption is rampant in China, and President Xi Jinping has presided over a much-publicised anti-graft campaign since coming to power in 2012, which some have compared to a political purge.