As the United States warns that two leading Chinese technology firms pose potential threats to its national security, an expert from a Chinese think tank has called this assertion "a barrier to trade."

"Technically or economically, it's impossible for Huawei and ZTE to place back doors deliberately," Fang Binxing, a member of the Chinese Academy of Engineering and president of Beijing University of Posts and Telecommunications, was quoted as saying by Guangming Daily on Wednesday.

"Back doors" refer to programs secretly inserted by developers, enabling attackers to install malicious software that could paralyze networks and allow hackers to gain entry into highly classified systems.

An 18-month White House-ordered review on Huawei, the world's second-largest maker of networking gear, indicated no evidence of Huawei espionage was found, but that it was still risky based on the presence of "back doors."

Fang said Huawei shares similar technology with Cisco in routers. As the world's leading maker of computer networking equipment, Cisco is able to find any back door without difficulty, if it exists.

"But now US companies and government agencies fail to provide any evidence," Fang said. "How can they still claim Huawei and ZTE insert back doors in their products?"

In recent years, Huawei has pursued a foothold in the US market as a private enterprise. Spying for China at the expense of its reputation "does not make sense," according to Fang.

Huawei and ZTE have already become Cisco's major competitors. But both of them hit an invisible wall when they attempted to expand their business in the United States.

Fang said US allegations against Huawei are groundless. "It just intends to set up trade barriers to Chinese companies," he said.

He also called on the Chinese government to tighten its own information security measures and establish a dedicated agency for this task.

According to a report released in early July by China's primary computer security monitoring network, nearly 50,000 overseas-based internet protocol addresses were involved in attacks against 8.9 million Chinese computers last year.

As China is facing increasing cyber attacks and threats from overseas, "we need a specific agency to take charge of network security issues," Fang said.