Emails sent by the founder of infidelity website AshleyMadison.com appear to have been exposed in a second, larger release of data stolen from its parent company, reports say.

Key points: Hackers dump a second set of data from Ashley Madison

Hackers dump a second set of data from Ashley Madison Second release exposes emails from Ashley Madison founder

Second release exposes emails from Ashley Madison founder Former American reality TV star Josh Duggar caught up in scandal

Former American reality TV star Josh Duggar caught up in scandal US defence investigates use of military email addresses

The second data dump appeared to include email messages linked to Noel Biderman, the site's founder and chief executive officer of its parent company Avid Life Media.

According to online technology site Motherboard, the hackers, who call themselves the Impact Team, said in a message: "Hey Noel, you can admit it's real now."

The release came as the US defence department said it was investigating the alleged use of military email accounts on the site.

David Kennedy, founder and security consultant at TrustedSec, said that the fresh release appeared to be authentic.

"Everything appears to be legit," he said in an email.

"We have portions downloaded and its confirmed legitimate thus far."

Former reality TV star Josh Duggar was among those caught up in the scandal, admitting he cheated on his wife after reports that he had subscribed to Ashley Madison.

Duggar, 27, a former campaigner for family values who appeared on the TLC show 19 Kids and Counting apologised for being "the biggest hypocrite ever" in a statement posted on his family's website.

The Motherboard report said the second release bore the hallmarks of Tuesday's dump.

The additional release will likely ratchet up the pressure on the company, which has been quiet about exactly how much and what sort of data was stolen in a breach in July.

Avid Life did not immediately respond to requests for comment.

"These guys are very diligent about not being caught," said Erik Cabetas, managing partner of Include Security, who has done forensic work on the initial dump.

The 20-gigabyte data dump reported on Thursday would be roughly double the size of the earlier one.

The earlier dump included personal details of millions of members of Ashley Madison, whose tagline is "Life is short. Have an affair".

Despite the negative publicity surrounding the cyber attack, demand for Ashley Madison's services has been steady since the data breach first announced in July, said Mark Brooks, CEO of internet dating consultancy Courtland Brooks.

"I would have thought this would be a death knell for that company because their entire business basis is privacy," Mr Brooks said.

"It just goes to say that all press is good press ... The awareness of the brand is through the roof."

US defence investigates use of military email addresses

The data release could have severe consequences for US service members if found to be real.

Several tech websites reported that more than 15,000 email addresses were government and military ones.

The Pentagon said it was aware of reports that military email addresses were among those posted earlier in the week.

Defence secretary Ash Carter told a Pentagon news conference that different service branches were looking into the matter.

"I'm aware of it. Of course it's an issue because conduct is very important," Mr Carter said.

"We expect good conduct on the part of our people.

"The services are looking into it, as well they should be, absolutely."

US military culture emphasises faithfulness to family and family life, values seen as particularly important in an environment where troops often have to spend months deployed away from their spouses and children.

Adultery is a violation of military law but it is difficult to prosecute because of the high standards of evidence required.

Reuters