The chief executives of AT&T (T.N) and Verizon Communications (VZ.N) rejected a request to delay the planned Jan. 5 introduction of new 5G wireless service over aviation safety concerns but offered to temporarily adopt new safeguards.

U.S. Transportation Secretary Pete Buttigieg and Federal Aviation Administration chief Steve Dickson had asked AT&T CEO John Stankey and Verizon CEO Hans Vestberg late Friday for a commercial deployment delay of no more than two weeks.

The wireless companies in a joint letter on Sunday said they would not deploy 5G around airports for six months but rejected any broader limitation on using C-Band spectrum. They said the Transportation Department proposal would be “an irresponsible abdication of the operating control required to deploy world-class and globally competitive communications networks.”

The aviation industry and FAA have raised concerns about potential interference of 5G with sensitive aircraft electronics like radio altimeters that could disrupt flights.