A new front has opened in the battle between the U.S. and China over control of global networks that deliver the internet. This one is beneath the ocean.

While the U.S. wages a high-profile campaign to exclude China’s Huawei Technologies Co. from next-generation mobile networks over fears of espionage, the company is embedding itself into undersea cable networks that ferry nearly all of the world’s internet data.

About 380 active submarine cables—bundles of fiber-optic lines that travel oceans on the seabed—carry about 95% of intercontinental voice and data traffic, making them critical for the economies and national security of most countries.

Current and former security officials in the U.S. and allied governments now worry that these cables are increasingly vulnerable to espionage or attack and say the involvement of Huawei potentially enhances China’s capabilities.

Huawei denies any threat. The U.S. hasn’t publicly provided evidence of its claims that Huawei technology poses a cybersecurity risk. Its efforts to persuade other countries to sideline the company’s communication technology have been met with skepticism by some.