AT&T Inc has promised to give Deutsche Telekom $6 billion in assets, services and cash as a break-up fee if U.S. regulators reject its proposed $39 billion purchase of the German company's T-Mobile USA, according to sources familiar with the matter.

The $6 billion would include $3 billion of cash, as AT&T has previously disclosed, and about $2 billion worth of spectrum and a roaming agreement valued at $1 billion, according two sources who asked not to be named as those details were not public.

While the cash agreement is already unusually high at 7.7 percent of the total deal price, the addition of assets and services of a similar value would mean that the companies are breaking global records with a 15.4 percent break-up fee, according to Thomson Reuters Data.

The high fee underscores AT&T's confidence that it can convince regulators to approve the deal, which is already being heavily criticized by many consumers and AT&T rivals including No. 3 U.S. mobile service Sprint Nextel .

The acquition of T-Mobile USA, ranked No. 4, would make AT&T, currently the No. 2 U.S. mobile service, the leader of the U.S. market, enabling it to leapfrog bigger rival Verizon Wireless, a venture of Verizon Communications and Vodafone Group Plc .

The deal needs approval from the U.S. telecommunications regulator, Federal Communications Commission, and the Department of Justice, which examines antitrust issues around mergers.

AT&T's chief executive, Randall Stephenson, had to defend the deal at a hearing held by skeptical lawmakers in Capitol Hill on Wednesday.

(Writing by Sinead Carew; editing by xxx)