US President Barack Obama speaks during the League of Conservation Voters Capitol Dinner at the Ronald Regan Building and International Trade Center on June 25, 2014 in Washington, DC

US President Barack Obama mocked Republicans who reject global warming as a "liberal plot," as he pumped up a crowd of environmental activists in Washington.

"Folks will tell you climate change is a hoax or a fad or a plot," Obama said at a League of Conservation Voters dinner.

"It's a liberal plot," Obama said, sarcastically.

"Many who say that actually know better and they're just embarrassed, they duck the question.

"'They say, hey, I'm not a scientist,' which really translates into, 'I accept that manmade climate change is real, but if I say so out loud, I will be run out of town by a bunch of fringe elements that thinks climate science is a liberal plot.'"

Obama compared climate change deniers to patients who ignore the science that shows they have a disease.

"I'm not a doctor either—but if a bunch of doctors tell me that tobacco can cause lung cancer, then I'll say, okay.

"I mean, it's not that hard," Obama said.

The US president is turning increasingly sardonic about lawmakers, not exclusively Republicans, who thwarted White House efforts during his first term to introduce a cap-and-trade system to regulate carbon emissions.

Some of Obama's foes either doubt the science behind climate change or fear that doing something about it will involve draconian measures that will hike energy prices or harm the still fragile US economy.

Obama recently unveiled a plan to use his executive powers to cut carbon emissions from US power plants by 30 percent from 2005 levels by 2030.

The plan would let US states choose their own approaches as long as each enforces restrictions on carbon emissions.

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© 2014 AFP