Apple Inc said it sold more than 10 million iPhones in the first weekend after its new models went on sale on Friday, underscoring strong demand for phones with larger displays.

Chief Executive Tim Cook said the company could have sold even more if supplies had been available.

Analysts had estimated first-weekend sales of up to 10 million iPhone 6 and iPhone 6 Plus models, based on record first-day pre-orders.

The new phones, which went on sale in 10 countries, drew more than 4 million pre-orders on Sept. 12, far surpassing the 2 million orders for the iPhone 5 model in the same period two years ago.

Apple shares were up 0.8 percent at $101.84 in early trading on the Nasdaq on Monday.

"While our team managed the manufacturing ramp better than ever before, we could have sold many more iPhones with greater supply and we are working hard to fill orders as quickly as possible," Cook said in a statement.

Last year, Apple sold 9 million iPhone 5S and 5C models in 11 countries, including China, in the first weekend they were available.

Sales of the latest models in China, the world's largest smartphone market, have been delayed by regulatory issues.

Apple sold 5 million iPhone 5 handsets, 4 million iPhone 4S models and 1.7 million iPhone 4 units during the first weekend.

The company said last week that many U.S. customers would have to wait until next month for their new iPhones due to strong demand.

Apple routinely faces iPhone supply constraints, particularly in years that involve a re-design.

The new iPhones will be available in 20 more countries on Sept. 26 and 115 countries by the end of the year, Apple said.

The company also said "Apple Pay" - its new wireless payment service that allows users to pay for items in stores with their phones - will be available in October to customers in the United States as a free update with the iOS 8 mobile operating system.

(Reporting by Soham Chatterjee in Bangalore; Editing by Ted Kerr)