Citigroup Inc. expects to double digital-banking users in Asia in the next five years, helping the third-biggest U.S. lender by assets boost client accounts and revenue without the cost of setting up branches.

The 7.5 million customers using Citigroup's online and mobile-banking services in the region will probably grow to 15 million by 2019, bolstered by rising affluence and a burgeoning middle class, Jonathan Larsen, Asia-Pacific head of consumer banking, said in an interview in Hong Kong last week.

"Our strategy is essentially an urban strategy and it's tapping into this new urban middle class that has been created in the last 30 years in Asia," said Larsen, 48.

Chief Executive Officer Michael Corbat is expanding in faster-growing markets while trimming brick-and-mortar branches and operations in locations with less potential. Asian consumer banking made up almost 10 percent of Citigroup's $59 billion of revenue in the nine months to September.

Larsen expects the number of outlets in Asia to stay little changed at 450 in coming years as the development of its digital network allows New York-based Citigroup to pursue business without relying on new outlets.

Citigroup said in October that it will exit consumer banking in 11 markets including Japan and consumer finance in Korea to boost returns. The new structure will serve almost 57 million clients in 24 markets, half of which are in Asia.

The Asian consumer business has 35 million customer accounts. It generated revenue of $5.76 billion for the nine months through September, accounting for 20 percent of the total for global consumer banking, company filings show.

The firm is renewing the systems that support its retail bank to develop a consistent technology platform across its markets, Larsen said. Citigroup opened its first Asian offices in 1902 in cities from Hong Kong to India.

The business that Larsen oversees encompasses a region where economic growth is outpacing the rest of the world. Developing Asian countries including China and India may expand 6.5 percent this year, more than triple the 1.8 percent rate for advanced economies, according to October projections from the International Monetary Fund.

"With the actually relatively buoyant outlook for the Asian economy over the next few years, with the continuing growth in the middle class, the continuing rise of consumer affluence, the macro-trend in market growth is not going to go away," Larsen said. "We are pretty optimistic about the outlook."