HONG KONG, Sept 18 (Reuters) - The Hong Kong Monetary Authority sold HK$5.425 billion ($700 million) into the market after the local currency hit the strong end of its trading band, according to data released by the HKMA.

The aggregate balance - the key gauge of cash in the banking system - will increase to HK$221.342 billion ($28.56 billion) on September 22, HKMA said on Friday.

The Hong Kong dollar is pegged in a narrow range of 7.75-7.85 to the U.S. dollar. The HKMA has been conducting sales in recent sessions to weaken the currency to keep it within that trading band.

($1 = 7.7500 Hong Kong dollars) (Reporting by Meg Shen and Donny Kwok; Editing by Toby Chopra)