The ABC network has refused to air an advert produced by Al Gore's environmental group, ruling that its charge of US government favouritism to the oil industry is too "controversial" for television.

The TV commercial, part of the WE campaign run by Gore's Alliance for Climate Protection, was submitted for airing after this week's presidential debate between Barack Obama and John McCain - both of whom have vowed to limit greenhouse gas emissions if elected.

But ABC concluded that the advert violated its internal policy against "controversial" content during network-sponsored programmes, network spokeswoman Julie Hoover told the Guardian.

"All of our advertising is reviewed on a case-by-case basis, and the context of this particular ad was determined not to be acceptable per our policy on controversial issue advertising," Hoover said.

The WE campaign has since attracted more than 170,000 supporters to an online petition drive asking ABC to reconsider its decision.

The script of the advert is similar in tone to political speeches made by Obama and McCain during the election season. An unseen narrator states that climate change can be combated through wind and solar power as well as "end[ing] our dependence on foreign oil".

Over an image of a young child playing with blocks, the narrator continues: "So why are we still stuck with dirty and expensive energy? Because big oil spends hundreds of millions of dollars to block clean energy. Lobbyists, ads, even scandals. All to increase their profits, while America suffers."

An ABC email published on the blog of Grist magazine stated that the advert was rejected due to its split-second shot of the US Capitol building.

"Per our guidelines, national buildings may be used in advertising provided the depictions are incidental to the advertiser's promotion of the product or service," the email stated. "Given the messages and themes of this commercial, the image of the Capital [sic] building is not incidental to this advertising."

Cathy Zoi, chief executive of the WE campaign, called ABC's decision "outrageous" in light of US networks' frequent airing of adverts from Chevron, Exxon Mobil and other oil companies.

"As our country faces deep economic problems, we need to be able to have an honest debate about the root causes of our problem," Zoi wrote in an email to supporters of Gore's group on Wednesday.

To build publicity for their products, American companies often produce TV adverts with content that pushes the limits of broadcast standards. A Snickers commercial featuring two men embarrassed after sharing a kiss was pulled from the US airwaves last year after complaints from gay-rights groups.

But rejection of an advert from a non-profit group is a far more rare occurrence. At the height of the US controversy over same-sex marriage in 2004, CBS and NBC turned down a commercial from the United Church of Christ that touted its acceptance of gay congregants.