WASHINGTON, June 10 -- The United States is preparing to pull the plug on the last direct connections between its telecom networks and China's three state-owned carriers, and some of the sharpest warnings about what that would break are coming from American companies, not Chinese ones.

The Federal Communications Commission is weighing a proposal that would bar US operators from interconnecting with China Unicom, China Mobile and China Telecom, the firms Washington has spent half a decade pushing to the edges of its communications system. A broader version under review would go further, stopping Chinese telecom companies that run data centers and internet exchange points inside the United States from handing traffic to other networks at a...