COPENHAGEN, Dec 27 (Reuters) - Novo Nordisk, the world's biggest insulin maker, is confident about its long-term growth plans but needs to secure approval for its new long-acting insulin Tresiba in the United States, its chief executive said in a newspaper interview on Saturday.

"When we look at the long-term plan ... we think that we can demonstrate growth, which is near the ten percent per year," Chief Executive Lars Rebien Sorensen told Danish daily Berlingske. "But it gets a little challenging in the short term, for it assumes, among other things, that we get Tresiba approved in the U.S."

Novo Nordisk's once-daily insulin Tresiba, the company's big hope for the future, failed to win U.S. approval last year.

"If we gets Tresiba approved in the U.S. I think we will be able to grow on average 10 percent a year over the next five years," Sorensen said.

The company has delivered annual growth of around 10 percent over the last decade but challenges in the United States have increased, he said.

"The question is how many new generations of insulin ... can be developed," he told the newspaper.

Sorenson pointed to competition in diabetes treatments from rivals such as Sanofi and Eli Lilly.

(Reporting by Ole Mikkelsen. Editing by Jane Merriman)