Actor Mark Ruffalo placed on terror watch list for gas drilling documentary.

Hollywood star Mark Ruffalo has been placed on a terror watch list by U.S. officials after organizing screenings for a new documentary about natural gas drilling.

The Zodiac actor arranged showings for GasLand earlier this year and spoke out about the dangers to the national water supplies.

But his efforts to raise awareness and demand a stop to natural gas drilling attracted the attention of officials from Pennsylvania's Office of Homeland Security.

He later discovered his efforts had landed him on a terror alert watchlist.

The 43-year-old actor, who also stars in this year's critical hit The Kids Are All Right, will not be stopped from flying.

Instead, he could face further security checks from TSA airport officials.

Gasland, directed by Josh Fox, follows the filmmaker as he visits communities in Pennsylvania where natural gas has been drilled.

Fox decided to document his trip after a natural gas company wrote to him in 2008 offering to lease his family's land in Milanville, Pennsylvania, for $100,000 (£64,000).

The 2010 documentary attracted attention for a particular scene in which a local man shows that he is able to set his tapwater on fire.

Others claim they are suffering from numerous health issues due – they believe – to their water having been contaminated.

The film was opposed by the natural gas industry.

Ruffalo told a US magazine he wasn't worried about being labelled a security threat.

He told GQ magazine, '(It's) pretty f**kin' funny.'

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