The Obama administration on Wednesday filed to block AT&T's proposed acquisition of wireless rival T-Mobile USA because of anticompetitive concerns, two sources familiar with the matter said.

A Justice Department spokeswoman declined to comment as did a spokesman for AT&T.

Blocking the combination on antitrust grounds would be a big blow to the telecommunication giant's expansion plans.

(Reporting by Jeremy Pelofsky. Editing by Robert MacMillan)