Natalie Portman romantic comedy tops US box office

Natalie Portman and Ashton Kutcher star as friends in a 'no strings' relationship

Natalie Portman romantic comedy No Strings Attached has topped the North American box office in its opening weekend.

The film, which also stars Ashton Kutcher in the story of two friends who fall in love despite their "no strings" relationship, took $20.3m (£12.7m).

Last week's number one, comic book adaptation The Green Hornet, slipped one place with $18.1m (£11.3m).

Vince Vaughn comedy The Dilemma was at three taking $9.7m (£6.1m).

No Strings Attached's opening weekend takings exceeded the modest expectations of the Paramount Pictures film, which cost $25m to make (£15.7m).

Paramount said women accounted for 70% of the audience, and cinema-goers aged under 25 gave it the best reviews in exit polls.

"It's a so-called 'chick flick' because of the subject matter, but it had appeal for both men and women with the whole idea of being 'friends with benefits'," Hollywood.com analyst Paul Dergarabedian said.

"For it to do $20 million is fairly impressive, considering it's rated R and January's not a hotbed of huge grosses, unless you're Avatar," he added.

Colin Firth's royal drama The King's Speech held steady at number four with $9.2m (£5.8m), while western True Grit fell two to five with $8m (£5m).

Portman has a second film in the top ten - ballet thriller Black Swan, which stayed at number six, bringing its total box office haul to $83.6m (£52.4m).

Christian Bale boxing drama The Fighter rose two places to seven, comedy sequel Little Fockers fell to eight, while a film based on cartoon character Yogi Bear was at nine.

Tron: Legacy rounded out the top 10.