HONG KONG, Sept 19 (Reuters) - The Hong Kong Monetary Authority sold HK$3.309 billion ($427 million) into the market in New York trading hours 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 by HK$8.734 billion, including HK$5.425 billion sold in Hong Kong trading hours, to HK$224.652 billion on Sept. 22, an HKMA spokesman said early on Saturday.

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 Donny Kwok; Editing by Daniel Wallis)