French upstart telecommunications company Iliad SA has made an offer for T-Mobile US Inc., in a bold bid to counter an offer for the country’s fourth-largest wireless carrier by Sprint Corp., said people familiar with the matter.

The contours of the offer, which was made less than a week ago to T-Mobile’s board, are unclear, but one of the people said it involves control of the U.S. company. It’s unclear what, if any, the response to the bid has been.