The Trump administration has warned more than 1,100 Environmental Protection Agency employees who work on climate change, reducing air pollution, enforcing environmental laws and other programs that they could be fired at any time.

An email, reviewed by The New York Times, was sent to staff members who were hired within the past year and have probationary status. Many of those employees were encouraged to join the E.P.A. under the Biden administration to rebuild the agency, which had been depleted during President Trump’s first term. Others are experienced federal workers who had taken new assignments within the agency.

Many had been hired to work on programs that Congress created through two recent laws, doing things like helping communities replace lead pipes, mediating toxic sites and funding clean energy projects aimed at reducing the greenhouse gas emissions that are heating the planet.

“As a probationary/trial period employee, the agency has the right to immediately terminate you,” the email states.

Molly Vaseliou, an E.P.A. spokeswoman, said in a statement that “our goal is to be transparent.” She declined to answer questions about the email, though, including whether Lee Zeldin, the agency’s new administrator, intended to terminate employees and, if so, for what reason.

“On his first day in office, he engaged directly with career staff across E.P.A.’s headquarters — spanning two city blocks in downtown D.C. — listening to their insights and perspectives,” Ms. Vaseliou said. “Ultimately, the goal is to create a more effective and efficient federal government that serves all Americans.”

At 9:21 a.m. on Monday, E.P.A. employees received another email notifying them of that the agencywide intranet was out of service. Without the internal agency network, employees cannot access documents or other information needed for their jobs.

The email from E.P.A.’s Office of Mission Support reads “Access to work.epa.gov is current unavailable” and that technical specialists were working to resolve the issue. It was not immediately clear if the outage was related to efforts to reduce the work force.

Asked about the email, Ms. Vaseliou said “There was an outage.”

Other federal agencies have been directed by the Office of Personnel Management to submit lists of probationary employees, but E.P.A. workers appear to be the first to receive notice that they may be immediately dismissed.

Leaders at the American Federation of Government Employees, which represents unionized E.P.A. employees, called the move a clear attempt to gut an agency that Mr. Trump dislikes. Through the E.P.A., the Biden administration developed aggressive regulations to curb planet-warming pollution from power plants, automobiles and oil and gas wells.

“E.P.A. is at the center of the bullseye for President Trump’s vindictive purge of public servants,” said Michelle Roos, executive director of the Environmental Protection Network, a group of agency alumni.

She called it “the most chaotic and vindictive transition in the history of the Environmental Protection Agency.”

Two E.P.A. employees who received the email said it had caused them to rethink the Trump administration’s offer to federal employees to resign but be paid through the end of September. The employees, who spoke on the condition of anonymity because they are concerned about being fired, said they had initially dismissed that offer as untrustworthy.

Being informed that their jobs are particularly precarious has caused them to rethink their options, both said.

Probationary employees are considered easier to fire because they do not have the full range of civil service protections, but rules still exist, said Marie Owens-Powell, president of the federation of government employees union, which represents about 8,000 E.P.A. workers.

“There has to be cause, and the cause can’t be because you’re a Democrat and it’s a Republican administration,” Ms. Owens-Powell said. The union is consulting lawyers about whether the administration can fire workers based on a simple declaration of a change in agency priorities, she said.

Ms. Owens-Powell said the Trump administration had placed about 15 E.P.A. employees who were working on diversity, equity and inclusion efforts on administrative leave. She said the next likely targets are those working on environmental justice, or programs to help marginalized communities that suffer from disproportionate amounts of air and water pollution.

Staffing at the E.P.A. peaked in 2004 during the George W. Bush administration, when there were 17,611 employees, according to the agency. Those levels ebbed and flowed slightly, but began to take a sharp dip during the Obama administration amid Republican control of the House and Senate.

When Mr. Trump entered the White House in 2017, the E.P.A. had 15,408 employees. The following year that number dropped to 14,172 as political appointees reversed regulations, shut scientists out of decision-making and shrank the agency’s budget.

The Biden administration sought to reverse those losses. The agency currently employs about 15,130 people around the country, a level slightly higher than when Ronald Reagan was president.