Powerful religious drama 'Doubt' topped the nominations on Thursday for the Screen Actors Guild (SAG) awards, a key indicator of likely contenders at next year's Oscars.

The adaptation of the successful Broadway play scored five nods, including best actress for Meryl Streep, best supporting actor for Phillip Seymour Hoffman and best ensemble cast, the equivalent of the Oscars' best picture.

The acclaimed movie revolves around a nun, played by Streep, who confronts a priest (Hoffman) at a New York Catholic school in the 1960s who she suspects of molesting a student.

Along with 'Doubt', the nominees for the best ensemble cast award included three other period dramas 'Frost/Nixon', 'Milk' and 'The Curious Case of Benjamin Button'. The fifth nominee was 'Slumdog Millionaire'.

The Screen Actors Guild awards, which take place on 25 January next year, are regarded as an important form guide heading into the Academy Awards, held roughly a month later on 22 February.

Actors form the biggest single group amongst the roughly 5800 entertainment industry figures who decide the Oscars, accounting for roughly 1200 voters.

Elsewhere on Thursday, late Australian star Heath Ledger cemented his status as the favorite for the awards season's best supporting actor honours, earning yet another nomination for his portrayal of Batman's cackling arch-enemy the Joker in summer blockbuster 'The Dark Knight'.

Ledger, who died of a drug overdose in New York in January aged 28, has already secured a posthumous Golden Globes nomination for his performance and is regarded as favorite to land an Oscar.

Other supporting actors in contention include Hoffman for 'Doubt', Josh Brolin for 'Milk', Dev Patel for 'Slumdog Millionaire', and Robert Downey Jr. for 'Tropic Thunder'.

In other categories, Brad Pitt received a nod in the lead actor race for his performance as a man who ages in reverse in 'The Curious Case of Benjamin Button'. Pitt's rivals include Richard Jenkins for 'The Visitor', Frank Langella for 'Frost/Nixon', and Sean Penn for 'Milk'.

In the best actress category, Streep faces competition from Anne Hathaway for 'Rachel Getting Married', Angelina Jolie for 'Changeling', Melissa Leo for 'Frozen River', Kate Winslet for 'Revolutionary Road'.

Winslet earned a second nomination in the best supporting actress category for her role as a concentration camp guard haunted by her past in the 'The Reader'. Other nominees include Amy Adams and Viola Davis, for 'Doubt', Penelope Cruz for 'Vicky Cristina Barcelona', and Taraji P. Henson for 'Benjamin Button'.

Next year's SAG Awards are taking place amid bitter divisions within the union about a whether actors should vote to authorise a strike. Ballots are to be mailed to voters on 2 January with votes to be counted three weeks later.

Earlier this week more than 130 stars, including Tom Hanks, George Clooney and Matt Damon, spoke out against the possibility of a walkout.