Republicans have launched an attack on the Obama administration's powers to act on climate change, proposing a 17% budget cut to the Environmental Protection Agency.

In a follow-up strike, they repeatedly challenged the legal authority of the agency to regulate greenhouse gas emissions during a contentious hearing in Congress.

The proposed $1.6bn (£1bn) cut to the EPA was the largest in dollar terms of some 70 budgetary measures put forward by Republicans on Wednesday.

Lisa Jackson, the EPA chief, said the attacks were part of a larger Republican project to roll back years of environmental and safety protections.

"I think this is a serious effort to weaken the clean air act," she said.

Jackson and the EPA have emerged as prime targets for the Republican majority in the House of Representatives, who have promised to block what they say are "job-killing" regulations.

The energy and commerce committee is now under the control of a Republican, Fred Upton, who told a seminar this week he did not believe in man-made climate change.

A number of other Republicans declared the science on climate change was "mixed".

In other exchanges, Jackson was accused repeatedly of overstepping her legal authority by introducing limited regulations on greenhouse gas emissions earlier this year – despite a supreme court ruling that such actions fell within the remit of the EPA.

She turned back the charges, pointing to the health costs and other dangers of climate change.

"I am not here to tell your committee that greenhouse gases are not a problem," she told the committee.

Her first appearance before the committee had been keenly awaited as Republicans and Democrats prepare to do battle over environmental regulations.

On Tuesday, Henry Waxman, the California Democrat who had earlier headed the committee, circulated a letter in which even the EPA chief in the George Bush era conceded that the agency had a legal obligation to act on climate change.

In addition to the call for the $1.6bn cut to the EPA, Republicans proposed axing $1bn for high-speed rail, $900m for energy efficiency, and paring the budget for the National Oceanic and Atmospheric Agency.