

David Zaslav, chief executive of Discovery Communications, says he's concerned about a Comcast-TWC merger. (Scott Eells/Bloomberg)

Discovery Communications' chief executive said Wednesday that the cable television programmer has expressed concerns to federal regulators about Comcast's $45 billion bid to buy Time Warner Cable.

If the merger is approved, Comcast would control 17 of the top 20 markets for advertisers, said Discovery CEO David Zaslav at the Goldman Sachs Communacopia investor conference.

The merger "presents some serious issues. We've had some discussions with the FCC about our concerns," Zaslav said during a question-and-answer session. "We’re just trying to look at Comcast owning 17 of the top 20 markets, including New York and L.A.," and what that means for the company's ability in "reaching advertisers and reaching scale."

He didn't disclose details of the company's conversations with regulators. The firm hasn't formed a final opinion on whether the merger should be rejected.

Instead, he said Discovery will look closely at proposed conditions and whether those will be enough to ensure competition for programmers.

"It will be a long process, we think the summer of 2015," Zaslav said.

Comcast has defended its relationship with programmers, saying it carries 160 independent programmers on its cable network. It also says that after the merger, it would operate in 16 out of the top 20 markets.

Discovery is one of the fastest-growing cable networks, which includes the Discovery Channel, Animal Planet and the lifestyle channel TLC. Zaslav said most of the company's growth is overseas, where international cable viewership is growing. Discovery Kids is the top network in Brazil, for example. That international growth has offset flat cable viewing in the United States, he said.