(Reuters) - AIA Group Ltd (>> AIA Group Ltd), the world's third-largest life insurer by market value, clocked a 20 percent increase in new business in the third quarter aided by strong sales in its main markets of China and Hong Kong.

China and Hong Kong together account for about half of new business growth globally at AIA, originally founded in Shanghai nearly 100 years ago and the first foreign insurer to be granted a license in China.

AIA said the value of new business, which measures expected profits from new premiums and is a key gauge for future growth, rose to $824 million in the quarter, just ahead of an average estimate of $813 million from three analysts polled by Thomson Reuters I/B/E/S.

Shares in AIA were, however, down 2.3 percent in Hong Kong morning trade, bucking the broader rising market. The stock has gained as much as 32 percent in the last three quarters.

The Hong Kong insurer last month agreed to buy the insurance unit of Commonwealth Bank of Australia (>> Commonwealth Bank of Australia) for $3.1 billion, in the biggest Asian buyout of an Australian financial firm.

The acquisition will help AIA, which had free surplus cash of nearly $11 billion as of end May, diversify its main markets with Hong Kong and China together accounting for about half of new business growth now at the insurer.

The insurer's regional business presence also includes Thailand Singapore, Malaysia and South Korea.

AIA, which listed in Hong Kong in 2010 after a spin-off from bailed-out U.S. insurer AIG, said China continued to be the insurer's fastest growing business in the third quarter while Hong Kong delivered double-digit new business growth.

Asia is a battleground for insurers such as AIA, Sun Life Financial (>> Sun Life Financial Inc) and a host of local players attracted by the region's lower insurance penetration levels and faster growth rates for insurance premiums than in the Western markets.

(Reporting by Shashwat Pradhan in Bengaluru and Sumeet Chatterjee in Hong Kong; Editing by Stephen Coates and Edwina Gibbs)