House Republicans warned Defense Secretary Jim Mattis on Wednesday that lawsuits filed against the Pentagon by environmental advocacy groups in the U.S. may be unwittingly helping America’s adversaries, particularly China.

A letter sent by members of the House Committee on Natural Resources warned that “while some lawsuits represent sincere and justified concerns about the effect of federal actions on the environment, others may be maliciously filed to stop, restrict, delay, or impose additional costs on U.S. military activities.”

Committee Chairman Rob Bishop, R-Utah, and Subcommittee on Oversight and Investigations Chairman Bruce Westerman, R-Ark., pointed specifically to U.S. engagement in the Asia-Pacific region, noting the military has faced lawsuits dating back decades alleging environmental violations.

They wrote that the Navy has been restricted in testing and training due to litigation, citing lawsuits led by the Natural Resources Defense Council focusing on how marine life are impacted by the Navy’s use of active sonar and underwater explosives.

“Active sonar is the most effective means of detecting the ultra-quiet diesel electric submarines deployed by foreign navies, such as China’s People’s Liberation Army Navy,” the letter noted.

The letter also cited lawsuits by Earthjustice that have hindered live-fire exercises in Hawaii and a fight by environmental groups to prevent the relocation of a Marine Corps Air Station in Okinawa, Japan to a less densely populated area.

The lawmakers claimed that for America’s adversaries, “such lawsuits may serve as an inexpensive tool or a military windfall which reduce U.S. defense capabilities.”

“Accordingly, environmental groups that bring such lawsuits may be knowingly or vulnerable to unwittingly serving as proxies for our foreign adversaries,” the letter states, before warning that such groups “could perform as self-funded, self-sufficient, and perpetual ‘launch and forget’ weapons.”

GOP LAWMAKERS RAISE RED FLAGS OVER ENVIRONMENTAL GROUP'S TIES TO CHINA

The committee requested the Pentagon provide related information and documents, including those that identify foreign entities working to use U.S.-based groups as “proxies” in litigation against the Pentagon.

The letter comes after the committee raised red flags about the NRDC’s advocacy work earlier this month, suggesting it is at risk of being coerced by China -- suggesting it may even need to register as a foreign agent. On Wednesday, the NRDC responded to the latest letter by saying it's work "serves the public interest."

"It is directed by our senior leadership and policy experts, accountable to our independent board of trustees and supported by millions of Americans. Any suggestion to the contrary is just false,” Bob Deans, director of strategic engagement for the group, said in a statement. “We can defend our country and protect our oceans at the same time -- in fact, that’s what the law requires. The agreements we’ve reached with the U.S. Navy help to ensure the security and readiness of our naval forces in ways that protect marine mammals from needless harm.”

Fox News’ Alex Pappas contributed to this report.