Screen icon Robert Redford has spoken out against a possible invasion of Iraq, calling on independent filmmakers to help prevent what he called the erosion of freedom of speech.

Speaking at the Sundance Film Festival, which he founded, he said the White House had "some explaining to do" to the American people before embarking on a war.

Warning that there were "threats around the edges" of United States freedom of expression, Redford said independent movies could help keep alternative artistic and political views alive.

Freedom of expression is our most precious virtue Robert Redford

Independent films "might carry a lot of weight in terms of how a government behaves, what information they are allowing out and what their attitude is about freedom of expression," he said.

"One of the virtues of independent films is not only that they needn't be burdened by commercial dogma, but that they shouldn't be hindered in expressing different points of view.

"Freedom of expression is our most precious virtue," he said. "We have to be on our guard not to be careless with it."

Anti-war cry

The star of Butch Cassidy and the Sundance Kid and All The President's Men said he could not ignore politics with anti-war protests flaring across the US.

"There is clearly a rising swell demanding answers to something so consequential," he said.

Clooney says dialogue is needed

"Whether that happens in time to stop something that should or should not happen... all I know is, I certainly haven't heard the answers.

"There's a lot of attitude, but not much policy, coming to the American people to tell them how they should know what to do."

Redford declined to elaborate, telling reporters he "really didn't want to get into the Iraq issue".

Redford's comments come just a day after actor George Clooney said that President George W Bush was running the US government "like the Sopranos".

Clooney accused the president of cutting a deal with France and Russia to ensure the United Nations Security Council would not complain when "we go into a war and kill a lot of innocent people".

Clooney and Redford are the latest stars to add their voice to the growing anti-war cry.

Last month more than 100 entertainers including Martin Sheen, Kim Basinger and Samuel L Jackson sent a letter to President Bush urging him not to start a war.