Bosses of the United States' law enforcement agencies have warned against using phones made by Chinese manufacturers Huawei and ZTE.
"We're deeply concerned about the risks of allowing a company or entity that is beholden to foreign governments that don't share our values to gain positions of power inside our telecommunications networks," FBI director Chris Wray told the Senate Intelligence Committee, tech site CNET reports.
Mr Wray says the two companies could give Chinese spies "the capacity to exert pressure or control over our telecommunications infrastructure".
"It provides the capacity to maliciously modify or steals information and provides the capacity to conduct undetected espionage."
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US telecommunications providers AT&T and Verizon have both pulled deals with Huawei in recent times, according to reports.
Asked by the panel if they would recommend people use Huawei or ZTE phones, none of the officials testifying to the committee - including Mr Wray and officials from the Central Intelligence Agency and the Defence Department - raised their hands.
Huawei said warning people against certain brands of phones was "misleading at best, dangerous at worst".
"Huawei is trusted by governments and customers in 170 countries worldwide, connecting one-third of the world's population," Huawei vice president of external affairs William Plummer said in a statement.
"Privacy and security are critically important to all of us these days and we must all be cautious to protect our personal and family and professional data from compromise."
Huawei was founded by Ren Zhengfei, a former engineer in China's People's Liberation Army.
"As a publicly traded company, we are committed to adhering to all applicable laws and regulations of the United States, work with carriers to pass strict testing protocols, and adhere to the highest business standards," ZTE said in response.
"ZTE takes cybersecurity and privacy seriously."
Lawmakers in the US are considering a Bill that would ban the two companies from bidding on government contracts.
Huawei is the world's second-biggest phone manufacturer, behind Samsung and ahead of Apple, but has struggled to get a foothold in the US.
Huawei helped build New Zealand's UFB infrastructure. The Green Party raised concerns when the contract was announced in 2012, but then-Communications Minister Amy Adams said there were no grounds for concern.