WASHINGTON — Attorney General William P. Barr said Thursday that China’s dominance of 5G telecommunications networks was one of America’s top national security and economic threats, amplifying warnings issued for years by intelligence officials but that President Trump has sometimes undermined.

Mr. Barr said that allowing China to establish dominance was not only a “monumental danger” as Beijing could use the technology for monitoring and surveillance, but also that “the stakes are far higher than that.”

“Our economic future is at stake,” Mr. Barr said in a speech delivered during a conference in Washington on threats that China poses to the United States. “The risk of losing the 5G struggle with China should vastly outweigh other considerations.”

Mr. Barr noted that two Chinese telecom giants, Huawei and ZTE, account for about 40 percent of the global 5G infrastructure market, which is expected to serve as the backbone for trillions of dollars’ worth of economic and industrial activity in an increasingly digital global economy. 5G networks promise substantially faster network speeds and prospects for new commercial applications in multiple industries like transportation and health care.