South Africa is the setting for the alien-human confrontation

A film about extra terrestrials by a first-time director and a cast of unknowns has topped the US box office.

District 9, directed by Neill Blomkamp, took $37m (£22.5m) in its opening weekend, displacing GI Joe.

The Time Traveler's Wife entered the chart at three, with used car caper The Goods: Live Hard, Sell Hard another new entry at six.

District 9 was produced by Lord of the Rings maker Peter Jackson, and was helped by a strong marketing campaign.

Curiosity

The film, which is about a group of aliens forced to live in a South African ghetto, was advertised with posters on bus stops asking people to report non-humans, and bus stop seats marked for humans only.

According to Rory Bruer, Sony's head of distribution, the ploy stimulated people's curiosity.

"It did really pique their interest and drove them to the internet and elsewhere to discover what's going on," he said.

US BOX OFFICE TOP FIVE 1 District 9 - $37m 2 GI Joe - $22.5m 3 The Time Traveler's Wife - $19.2m 4 Julie & Julia - $12.4m 5 G-Force - $6.9m Source: Hollywood.com

Blomkamp, who also co-wrote the movie, previously made music videos and adverts.

District 9's largely unheralded cast includes South Africans Sharlto Copley and Mandala Gaduka.

Another new entry in the chart is animation Ponyo at number nine, from Hayao Miyazaki, who made Spirited Away, winner of the best animated feature Oscar in 2003.

But teen rock musical Bandslam, featuring High School Musical star Vanessa Hudgens, failed to make a big impression and landed outside the top 10 in its opening weekend.