"The EPA fired five whistleblowers who signed the EPA Declaration of Dissent, and issued a Notice of Removal for an additional four employees," according to a statement from Stand Up for Science, a non-profit organization created in response to US President Donald Trump's cuts to federal research funding and other policies.

The EPA appeared to acknowledge the job losses in a statement on Friday, but did not specify if employees were fired.

"Following a thorough internal investigation, EPA supervisors made decisions on an individualized basis," an agency spokesperson wrote to AFP, declining to comment on "individual personnel matters."

Since taking charge of the federal agency tasked with ensuring clean air, land and water, EPA Administrator Lee Zeldin has executed Trump's agenda, including gutting climate regulations, ramping up fossil fuel development and slashing funding for clean energy.

The changes have drawn fierce backlash from scientists and environmental advocates alike.

The EPA suspended more than a hundred employees in July after they signed the scathing open letter accusing Zeldin of pushing policies hazardous to both people and the planet.

The letter -- signed by 270 employees with 170 choosing to be named -- described a climate of political interference and warned that the agency's leadership was eroding public health protections and scientific integrity.

On Friday, the EPA noted their "zero-tolerance policy for career officials using their agency position and title to unlawfully undermine, sabotage, and undercut the will of the American public that was clearly expressed at the ballot box last November."

"The petition -- signed by employees using a combination of their titles and offices -- contains inaccurate information designed to mislead the public about agency business," it added.

The Trump administration has similarly clamped down on federal employees who signed letters of dissent at other agencies, including the Federal Emergency Management Agency (FEMA) and the Centers for Disease Control and Prevention (CDC).