Whistle-blower group WikiLeaks says it has begun publishing more than five million emails from a U.S.-based intelligence company, which the group believes has been targeting WikiLeaks and its founder, Julian Assange.

The emails from Stratfor, which were apparently obtained by hackers, began appearing on the WikiLeaks website around midnight on Sunday.

"The material contains privileged information about the U.S. government’s attacks against Julian Assange and WikiLeaks and Stratfor’s own attempts to subvert WikiLeaks," the group said in a statement posted online. "There are more than 4,000 emails mentioning WikiLeaks or Julian Assange."

The group said the messages, which are dated between July 2004 and late December 2011, reveal "the inner workings" of a company that provides intelligence to clients including military contractors such as Lockheed Martin  and government agencies such as the U.S. Department of Homeland Security and the U.S. marines.

Assange held a news conference Monday at the Frontline Club in London, where he described Stratfor as "a private intelligence Enron," the energy giant whose spectacular bankruptcy turned it into a byword for corporate malfeasance.

Assange accused Stratfor of running a network of paid informants, monitoring activist groups on behalf of major multinationals and making investments based on its secret intelligence. But he refused to answer questions about how WikiLeaks obtained the emails.

Stratfor, which describes itself as a subscription-based "provider of geopolitical analysis," announced in December that its computer system had been breached. Subscribers' email addresses and credit card information were stolen, the company said, as well as internal emails.

The hacker group known as Anonymous claimed responsibility for the attack. 

Move called 'breach of privacy'

Stratfor said the messages appeared to be the same ones stolen by hackers over the Christmas holidays. The company called the emails being released a "deplorable breach of privacy" and claimed that some of the messages may have been "forged or altered."

Stratfor pushed back against the suggestion there was anything improper in the way it dealt with its contacts, saying the company "has worked to build good sources in many countries around the world, as any publisher of global geopolitical analysis would do."

The company also posted a message on the social media site Twitter disputing rumours that its founder and CEO, George Friedman, had resigned.

The small selection of emails initially published to WikiLeaks' website turned up a mix of the innocuous and embarrassing, but Assange promised more explosive material in the coming weeks.

One message described a $6,000 monthly payment made to a Middle Eastern source, another carried bits of gossip dropped by a retired spook, and many were filled with off-colour office banter.

WikiLeaks said it had partnered with more than 25 publications, including Rolling Stone in the U.S. and The Hindu in India, which are helping to sift through the trove of correspondence.

With files from The Associated Press