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The Hobbit: An Unexpected Journey has topped the US box office chart for a second week.

Director Peter Jackson's return to Middle Earth took $36.7m (£22.6m) over the weekend, dwarfing its competition.

It easily beat Tom Cruise's latest action movie, Jack Reacher, which came second with $15.6 million (£9.6m).

But a drop-off in The Hobbit's takings, down from $84.8m last week, suggests it will struggle to match the success of the final Lord Of The Rings film.

The Return Of The King made $1.2bn worldwide after its release in 2003. At the time, it was the second highest-grossing film ever, although it has since been overtaken by the likes of Avatar and Harry Potter and the Deathly Hallows - Part 2.

Box office analysts said the 57% decline in The Hobbit's second-week takings was due to the Christmas shopping rush.

"The real winner this weekend might be holiday shopping," said Paul Dergarabedian, from Hollywood.com.

Tom Cruise's film also opened with lower-than-expected takings, making approximately half as much money as Mission Impossible 4, which was released this time last year.

The movie is based on Lee Child's Jack Reacher character, a former soldier and military policeman who lives "off the grid" and doles out vigilante-style justice.

Written and directed by The Usual Suspects' Oscar-winning scriptwriter Christopher McQuarrie, the film has fared well with critics, despite initial worries about the Cruise, who is 5ft 7in tall (1,70m), playing the 6ft 5in hero of the novels.

Elsewhere in the box office, Judd Apatow's comedy This Is 40, a sequel to his 2007 hit Knocked Up, debuted at number three, taking $12m.

Barbra Streisand comedy The Guilt Trip was another new entry, at number six, while a 3D version of Pixar's Monsters Inc came in at seven.

A prequel to the hit movie, about two Monsters employed to scare small children, is due for release next year.

Meanwhile, a limited release for Kathryn Bigelow's political thriller Zero Dark Thirty fared particularly well.

Based on the story of the CIA's hunt for Osama Bin Laden, the Oscar-tipped movie was only shown in five cinemas - but managed to make $410,000 over the weekend.

Playing to packed houses, it raked in $82,000 per screen. By comparison, Jack Reacher only made $4,654 per screen.

With more Oscar bait due to hit US cinemas on Christmas Day - including Quentin Tarantino's Django Unchained and the film adaptation of Les Miserables - receipts are expected to rise next week.

"We haven't reached the key holiday play time yet," said Dan Fellman, head of distribution for Warner movies.

"It explodes on Tuesday and goes right through the end of the year."