Whistle-blowing website WikiLeaks has begun publishing what it says are millions of highly sensitive emails obtained from intelligence company Stratfor.

The initial release is from around 170 emails, out of a total of 5.5 million that WikiLeaks claims to have got its hands on. The website says the haul will reveal information about Stratfor, such as its "web of informers, pay-off structure, payment-laundering techniques and psychological methods."

Unlike previous WikiLeaks revelations, it appears that the site gained these emails after a hack by online warriors Anonymous. WikiLeaks’ usual method is to get information from an insider, such as the leak of US highly-classified US diplomatic cables in February 2010. In that case, the information was handed to WikiLeaks by Bradley Manning.

Stratfor fell victim to a widely-publicised hack at the end of 2011, which Anonymous claimed to have carried out. In that attack, 200GB of information was exposed, including email addresses belonging to subscribers as well as credit card information from customers. According to Reuters, victims included U.S. Vice President Dan Quayle, former Secretary of State Henry Kissinger and former CIA Director Jim Woolsey.

It appears the Anonymous attack also resulted in the emails being taken. One Twitter account said to be linked to the hacktivism group began publishing updates related to the release a few hours ago.

"We promised you those mails and now they’ll finally be delivered. Five million (that’s 5,000,000) emails at your pleasure," said one. "We’ll gladly accept the apologies of all those who claimed that no dirt would be in those mails. #Antisec," said another. "There’s a treasure trove of nasty details in those emails. We think there’s something for everyone," added a third.

While Stratfor has admitted the authenticity of the leak it has suggested that some of the emails may have been altered or faked. They date from 2004 to the end of 2011.

Some of the emails reveal information about payoffs and informants. WikiLeaks also suggests that "Stratfor did secret deals with dozens of media organisations and journalists" to gather information.

"While it is acceptable for journalists to swap information or be paid by other media organisations, because Stratfor is a private intelligence organisation that services governments and private clients these relationships are corrupt or corrupting," the WikiLeaks announcement said.

"This is a deplorable, unfortunate — and illegal — breach of privacy," said a statement from Stratfor CEO George Friedman. "Some of the emails may be forged or altered to include inaccuracies; some may be authentic."

The company said the release of the emails was a "direct attack on Stratfor. This is another attempt to silence and intimidate the company, and one we reject. [We] will not be silenced and will continue to publish the geopolitical analysis our friends and subscribers have come to rely upon."

Stratfor has been labelled a "shadow CIA" – a global intelligence firm that specialises in defence and government data. It describes itself as a "geopolitical analysis" corporation and, in one of the leaked emails, as a "publishing company" that "uses intelligence rather than journalistic methods to collect information."