WASHINGTON (Reuters) - NBC Nightly News anchor Brian Williams has been suspended without pay for six months after admitting last week that a story he told about coming under fire while on a helicopter during the Iraq war was not true, the network said on Tuesday.

"By his actions, Brian has jeopardised the trust millions of Americans place in NBC News," NBC Universal chief executive Steve Burke said in a network statement. "His actions are inexcusable and this suspension is severe and appropriate."

Mr Williams had struggled to explain in an interview with United States military newspaper Stars and Stripes how he often misrepresented an Iraq war story of his helicopter coming under fire.

In his first published interview, Mr Williams, the anchor of top-rated NBC Nightly News, said he assumed the helicopter took damage.

Mr Williams took himself off the air on Saturday as the Comcast Corp-owned network investigates his claims that he rode in a helicopter that was hit by a rocket-propelled grenade during the first days of the Iraq War in 2003. "It's very basic I would not have chosen to make this mistake," Mr Williams told the newspaper on Feb 4.

Stars and Stripes first reported on a number of soldiers who disputed the claims, saying Mr Williams was not on or anywhere near the helicopter that was hit.

The claims have threatened to erode the credibility of the 55-year-old, who has anchored "Nightly News" since 2004 and helped maintain its top spot among network newscasts for most of that time.

The scandal has also stoked a wider debate about the role of a news anchor in a world where the relevance of a nightly network broadcast has waned in the Internet age.

"Because I knew we had all come under fire, I guess I had assumed that all of the airframes took some damage because we all went down," Mr Williams said in the interview published on Monday. "I don't know what screwed up in my mind that caused me to conflate one aircraft from the other," he added.

Mr Williams' apology last week, in which he said he misremembered the event, was widely mocked and derided.

The backlash against Mr Williams first gained steam on social media after a Jan 30 "Nightly News" segment in which Mr Williams retold the false story.

Mr Williams had frequently told his version of events since first reporting it in March 2003, and his retellings over the years have often differed.

Unlike the buttoned-up public image of many network anchors, Mr Williams frequently appears on television away from his anchor chair, hosting NBC's late night sketch comedy show Saturday Night Live in 2007 and appearing often on shows like Jimmy Fallon's The Tonight Show on the network.

On Sunday, Mr Williams called off a scheduled Thursday appearance on David Letterman's Late Show.

Mr Williams is also facing scrutiny over statements he made about Hurricane Katrina in 2005.