Former Defense Secretary Donald Rumsfeld, in an interview with Fox News, faulted the Bush administration for failing to effectively make the case for Guantanamo Bay, saying the facility was and still is "one of the finest prison systems in the world."

The former Pentagon chief also defended the Bush administration's handling of the Iraq war, saying none of the top officials lied about weapons of mass destruction as a pretext for war, and he warned that the threat to the United States from Islamic extremism remains great.

Rumsfeld is speaking publicly about some of the administration's most critical decisions for the first time since stepping down in 2006 as he promotes his new memoir, "Known and Unknown."

The release of the book Tuesday has, true to form for the feisty veteran of Washington inner circles, generated plenty of controversy from groups and individuals challenging his version of events during those years.

But Rumsfeld is taking on the administration's critics.

"For people to say Bush lied, Colin Powell lied and Condi Rice lied or Cheney lied or Rumsfeld lied -- it's just not true," Rumsfeld told Fox News' Sean Hannity. Asked about former Secretary of State Powell's infamous 2003 speech to the United Nations in which he cited questionable intelligence about Iraq's alleged weapons program, Rumsfeld said Powell was an "honorable man" who "believed every word he gave in that speech."

Rumsfeld also defended the detention camp at Guantanamo Bay, which the Obama administration has been unsuccessfully trying to shutter for the past two years. Rumsfeld lamented that officials were never able to improve the camp's public image.

"It is an exceedingly well-run prison and the folks down there have done and are doing an excellent job," he said. "The heartbreaking thing with respect to Guantanamo is not that there's anything wrong with it. It's one of the finest prison systems in the world. What's awkward is the fact that for whatever reason the administration was incapable of persuading people that that was a first-class operation, that they were not torturing people, that they were not hurting people."

Rumsfeld described the prison camp as a "fine operation" and said the service members who operate it have "taken a lot of heat unfairly."

Rumsfeld also made clear that he has not dialed down his concerns about the threat posed by Islamic extremism.

"Let there be no doubt, there are extremists out there who are determined to do damage to the United States of America and to kill Americans and to impose a caliphate over a large fraction of this globe," he told Fox News. "And they have weapons of increasing lethality at their disposal and at their beck and call."

In his 800-page memoir, Rumsfeld covers everything from the early days of the Iraq war to the Abu Ghraib prison scandal to his eventual exit from the administration.

Rumsfeld is also planning to release a trove of documents on his website, www.rumsfeld.com, in the weeks to come. One of those documents, a 2002 briefing to the Joint Chiefs of Staff dealing with the "Iraqi WMD program," was posted Tuesday.

The briefing noted that intelligence at the time was sketchy. It said "our assessments rely heavily on analytic assumptions and judgment rather than hard evidence."

The briefing said Iraq has the knowledge to build a nuclear weapon "without external expertise" but that "we do not know the status of enrichment capabilities" or whether the country has purchased or attempted to purchase such a weapon. U.S. knowledge of the Iraqi nuclear program, the briefing acknowledged, was mostly based on "analysis of imprecise intelligence."