PHILADELPHIA, March 12 (Reuters) - Sirius Satellite Radio Inc Chief Executive Mel Karmazin said on Wednesday he hoped U.S. regulators would rule by the end of March on the proposed merger of Sirius and XM Satellite Radio Inc.

Speaking at the Bear Stearns 21st Annual Media Conference, Karmazin said he "took heart" about recent comments by U.S. Federal Communication Commission Chairman Kevin Martin, who indicated that his agency aimed to rule on the deal by the end of March.

That time frame could change, Karmazin cautioned, since regulatory approval of the proposed $4.2 billion merger of the two satellite radio companies had taken longer than initially expected. The companies had hoped regulators would sign off on the deal by the end of 2007.

"The fact that it has lingered this long, it has been interpreted ... as good news," Karmazin said.

"Clearly if there was a big problem with the merger, it wouldn't take the (regulators) this long to figure it out. Either you believe we compete with a whole bunch of audio choices or you think there's a distinct market called satellite radio," he said.

The Department of Justice appeared to have all of the information it needed on the deal, but the companies were having active discussions with the FCC, he said. Talks with the FCC had recently accelerated, Karmazin said.

Karmazin said the companies needed to convince regulators that the deal would be in the public interest and not harm competition, as some critics have claimed.

XM Satellite Radio Holdings Inc Chairman Gary Parsons echoed Karmazin's comments on Wednesday and said he was confident that the regulatory review was moving forward "in a timely manner."

"We believe that DOJ has the information and feel the FCC has the necessary information to complete their decision-making process and have confidence they will move forward and will complete that in a timely manner," Parsons said at the conference.

Unless the DOJ sued to stop the proposed merger, the deal could close immediately after the FCC ruled, Karmazin said.

Shares of Sirius, whose programming includes "shock jock" Howard Stern and NASCAR auto racing, gained 15 cents, or 5.5 percent, to $2.90 in afternoon trading on Nasdaq.

Shares of XM Satellite, whose programming features Oprah Winfrey and Major League Baseball, gained 61 cents, or 5.5 percent, to $11.70 on Nasdaq.

(Reporting by Jessica Hall; Editing by Brian Moss and Gerald E. McCormick)

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