The opening salvoes have been fired in a new political battle in the US over greenhouse gas emissions.

Having failed to pass legislation through Congress, President Obama wants the Environmental Protection Agency (EPA) to regulate emissions.

But draft measures before Congress seek to squash the EPA's authority.

Testifying to a congressional committee, EPA chief Lisa Jackson said the bill ran counter to science and counter to a 2007 Supreme Court ruling.

The issue could have ramifications for the UN climate negotiations, with many developing countries looking to the US - as the world's biggest economy - for leadership.

But the politicians behind the new bills cite domestic concerns as the reason for their move - in particular, concern that implementing recently announced regulations limiting emissions from power stations would hurt the economy.

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