BARCELONA, Spain (Reuters) - U.S. media group Time Warner is optimistic about 2011 as TV ad sales growth accelerates, and sees potential to increase subscriber numbers for its pay cable channel HBO in the United States.

Chief Financial Officer John Martin said on Wednesday U.S. TV subscription revenue should grow this year despite an expected loss of 1.5 million subscribers, many of whom had signed up thanks to promotions offering deep discounts.

"Yes, the environment remains competitive but the position that HBO occupies, we still believe, is very attractive," Martin told Morgan Stanley's annual technology, media and telecoms conference in the Spanish city of Barcelona.

"We're growing very aggressively internationally," he said. "We believe there's even subscriber growth potential inside the U.S.," he added.

Time Warner's warning this month about its expected loss of customers, part of a so-called cord-cutting phenomenon as customers fail to renew promotional contracts, spooked investors despite the fact that the company raised its full-year outlook.

Martin said television advertising was currently strong for the industry as a whole and for Time Warner in particular, and he expected an improvement in the rate of growth this quarter as price increases kicked in.

"On a year to date basis for the first 9 months of this year we're at about 12 percent, and we would anticipate that in the fourth quarter advertising is going to accelerate," he said.

"It's still early days to predict how next year's going to be but I feel really optimistic right now."

(Reporting by Georgina Prodhan; editing by Elaine Hardcastle)