AT&T Inc. and Verizon Communications Inc. rebuffed a request from federal transportation officials to voluntarily delay the launch of new 5G wireless services, extending a showdown that could lead to potentially disruptive U.S. flight restrictions as soon as this week.

The cellphone carriers on Sunday offered a counterproposal that would further dim the power of their new 5G service for six months to match limits imposed by regulators in France, giving U.S. authorities more time to study more powerful signals’ effect on air traffic. The companies had planned to launch the service Wednesday in as many as 46 of the nation’s largest metropolitan areas.