White House climate adviser Gina McCarthy is planning to step down, according to two sources familiar with the deliberations, likely ending a tenure marked by ambitious emissions targets but failure in securing major US carbon-cutting legislation.

McCarthy, 67, had initially planned to remain in the White House for about a year, hoping to help federal agencies implement President Joe Biden’s ambitious climate legislation, but those efforts stalled amid intraparty opposition from key Democratic senators, including Joe Manchin.

McCarthy has already delayed her departure, and told one Reuters source that she plans to leave as soon as next month.

White House spokesman Vedant Patel said on Thursday: “This is not true and there are no such plans under way and no personnel announcements to make.”

“Gina and her entire team continue to be laser focused on delivering on President Biden’s clean energy agenda,” he said in an email.

Multiple news outlets, including the New York Times and the Washington Post, separately reported that McCarthy had told confidantes that she was planning to leave her job in the coming months, with the Times reporting that she had said she was “frustrated by the slow pace of climate progress”.

A Politico poll from December found that 80% of Americans who labeled themselves left-leaning said that the Biden administration is doing too little to address climate change. McCarthy publicly responded to that sentiment in February, Politico reported, saying, “We understand people’s frustration. Would we all like to be running faster and faster? Yes, we would.”

The White House did not immediately respond to a request for comment from the Guardian on the reports of McCarthy’s plans.

McCarthy, a former Environmental Protection Agency administrator during the Obama administration, was selected by Biden to a new role leading domestic climate policy coordination at the White House. She serves as a domestic counterpart to John Kerry, who Biden appointed as his special international envoy on climate change.

Her deputy, Ali Zaidi, who served as a climate policy adviser in the Obama White House, is seen as her likely replacement, the Washington Post reported.

Biden came into office with an ambitious climate agenda, pegged to a $555bn plan to transition to cleaner energy in all aspects of American life. Before those policies stalled, McCarthy, a regulatory expert, was going to be tasked with implementing the plan across multiple agencies.

Biden had promised to wean the nation off fossil fuels, but has now found himself looking for ways to increase global supply of oil and other carbon-rich energy products amid the Russian invasion of Ukraine and high gas prices that advisers see damaging his standing with voters.

McCarthy’s position was a key demand by the liberal wing of the Democratic party and an illustration of Biden’s commitment to the cause. Not replacing her could be seen as a retreat by the environmental community.