Defense Secretary Ash Carter said the military services are looking into the leak of thousands of military emails registered to adultery website Ashley Madison.

"I'm aware of it, of course it's an issue, because conduct is very important. We expect good conduct on the part of our people," Carter said at a Pentagon briefing on Thursday.

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The military services are looking into it, "as well they should be," he said.

Adultery in the military is a prosecuteable offense under Article 134 of the Uniform Code of Military Justice. Maximum punishment includes dishonorable discharge, forfeiture of all pay and allowances, and confinement for one year.

However, any action would be determined after a review of all credible evidence, defense officials said.

On Wednesday, thousands of emails registered to the the website, which assists spouses seeking extramarital affairs, were revealed by hackers.

More than 15,000 of the email addresses were potentially hosted on government and military servers.