Director Ang Lee's sexually-explicit spy thrille Se, Jie (Lust, Caution) was the surprise winner of the top award at the Venice film festival on Saturday, just two years after he won with Brokeback Mountain.

The movie is a World War II thriller set in Shanghai featuring long, and sometimes violent, sex scenes.

The verdict means Asian directors have won the Golden Lion on the Lido waterfront for the past three years.

The Silver Lion for best director went to US filmmaker Brian De Palma, whose Redacted shocked audiences with its brutal reconstruction of the real-life rape and murder of a 14-year-old Iraqi girl by US soldiers in 2006.

Lee told the glitzy red-carpet prize ceremony that Lust, Caution took him to "some very difficult places".

"I have invited you to come along with me and in the end to stay down there with me," he added.

The film is Lee's return to the theme of forbidden love after gay cowboy hit Brokeback.

This time the setting is the teeming streets of 1940s Shanghai.

The film centers on a group of revolutionary students bent on killing a powerful political figure who collaborates with occupying forces during the War of Resistance Against Japanese Aggression.

First-time actress Tang Wei portrays the young woman who agrees to ensnare the sinister figure, played by one of Asia's biggest screen stars, Tony Leung.

The sometimes violent sex scenes between them were a major talking point in Venice, and Lee hinted to reporters they were real.

The main competition lineup of 23 movies was strong on political cinema, with two US entries tackling the war in Iraq alongside pictures about migrant workers in Britain, police brutality in Egypt and corporate corruption in New York.

Tunisian-born director Abdellatif Kechiche's immigration drama La Graine et le Mulet ("The Secret of the Grain") was the pre-award favourite for the Golden Lion, but took away one of two runner-up jury prizes.

Todd Haynes, one of six US productions in competition, scooped the other runner-up slot with I'm Not There, his conceptual biopic about singer-songwriter Bob Dylan.

In a bold piece of casting, Australian-born Cate Blanchett was one of six performers to play the singer at various stages of his life, and it paid off when she was named best actress in Venice.

Hollywood star Brad Pitt was the surprise winner of the best actor award for his portrayal of legendary outlaw Jesse James in The Assassination of Jesse James by the Coward Robert Ford. "I was really surprised and remain surprised," Pitt said at the Toronto International Film Festival.

Asked to explain controversial choices, jury president Zhang Yimou said: "Even if we had all the force and power of heaven, we would not have been able to give a verdict that would please everybody."