By Alina Selyukh and Sinead Carew WASHINGTON/NEW YORK (Reuters) – A pair of potentially transformative U.S. telecoms and cable deals could run afoul of Obama administration regulators who worry that mergers among market leaders would hurt consumers. With both cable and mobile phone operators grappling with slowing growth, speculation has intensified recently about potential takeovers of No. 4 wireless service provider T-Mobile US Inc and No. 2 cable service provider Time Warner Cable Inc. Some possible buyers, including Sprint Corp and Comcast Corp, may face headwinds in convincing U.S. regulators that their deals would improve competition. The Obama administration definitely is more skeptical of large corporate combinations… They are concerned about the effects of market concentration on consumers, said Robert McDowell, who stepped down as the senior Republican member of the Federal Communications Commission earlier this year.