HONG KONG, April 13 (Reuters) - Hong Kong's de-facto central bank bought HK$9.051 billion ($1.153 billion) from the market in New York trading hours to stop the local currency from weakening and breaking its peg to the U.S. dollar.

The Hong Kong dollar is pegged to a tight band of between 7.75 and 7.85 versus the U.S. dollar.

The aggregate balance - the key gauge of cash in the banking system - will fall to HK$57.164 billion on April 14, an HKMA spokesperson said on Thursday morning. (Reporting by Donny Kwok; Editing by Muralikumar Anantharaman)