The top senator on antitrust matters has written to NBC to express concern that some of its Internet coverage of the Winter Olympics is limited to pay television subscribers.

Senator Herb Kohl's letter said sports fans who want to see some of the Olympic coverage on the site must first register with the site after validating a subscription with your cable, satellite or IPTV provider.

I fear that this practice of locking up certain content only for pay-TV subscribers may be a preview of what is to come with respect to TV programing shown on the Internet, particularly in the context of the proposed Comcast/NBC Universal merger, Kohl wrote in a letter to NBC President Jeff Zucker.

Kohl, a Democrat from Wisconsin, chairs the antitrust subcommittee of the Senate Judiciary Committee.

NBC Universal, in an email statement, responded it was spending nearly a billion dollars to bring the Vancouver Olympics to U.S. fans in on broadcast television, cable and the Internet.

This three-part offering has been structured to provide the financial support to help justify that investment, and bring U.S. fans the high-quality, professionally produced content they demand, the company said in a statement.

Under the terms of the Comcast/NBC Universal deal, cable giant Comcast plans to buy 51 percent of NBC Universal, now owned by General Electric.

NBCU also owns cable TV networks, a movie studio and theme parks. It also owns a stake in online video site

The proposed deal has been heavily criticized by public interest groups, warning that consumers could ultimately be hurt if Comcast, the biggest U.S. cable and broadband provider, also controls substantial amount of entertainment content.

Kohl, in his letter, asked NBC why it opted to require a pay-television subscription rather than charging consumers directly and whether any pay television companies had contributed financially to, and if that was why NBC had the sign-in requirement.

(Reporting by Diane Bartz; Editing by Tim Dobbyn)