WASHINGTON (Reuters) - Bidding reached $18.55 billion on Friday in the U.S. Federal Communications Commission's record-setting auction of government-owned wireless airwaves, but there were no new offers for two large, closely watched slices of spectrum.

The total bidding, which covers five separate blocks of spectrum in the auction, was up from $15.64 billion on Thursday.

There were no new bids on a major slice of the airwaves, known as the "C" block, which will have to be made accessible using any device or software application, under FCC rules. A bid of $4.71 billion, made on Thursday morning remained the top offer.

Nor were there any new bids Friday on a nationwide piece of the spectrum, known as the "D" block, which must be shared with public safety agencies under auction rules set by the agency. A bid of $472 million from last week still stood.

The lone $472 million bid for the D block spectrum, which came in the first round of the auction a week ago, is far below the $1.3 billion minimum price set by the FCC. If bidding fails to reach the minimum, the FCC will have to decide whether to re-auction the D block or possibly modify the network-sharing requirement.

The open-access condition on the C-block spectrum is important because U.S. wireless carriers have traditionally restricted the models of cell phones that can be used on their networks and limited the software that can be downloaded onto them, such as ring tones, music or Web browser software.

But AT&T and Verizon began moving away from that restrictive stance in recent months.

The FCC is keeping bidders' identities secret until the entire auction ends. But analysts say Verizon Wireless and Internet search leader Google are the most likely bidders for the C block.

The 700-megahertz signals are valuable because they can go long distances and penetrate thick walls. The airwaves are being returned by television broadcasters as they move to digital from analog signals in early 2009.

In addition to the C and D blocks, the other spectrum includes more local chunks set aside in blocks designated "A" and "B". The final, "E" block, is considered less useful because it is limited to one-way data transmission.

The electronic auction will end when no more bids are submitted.

(Reporting by Peter Kaplan, editing by Leslie Gevirtz)

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