FRANKFURT, Oct 31 (Reuters) - Greek banks have a capital shortfall of 14.4 billion euros ($15.9 billion) if the country's economy deteriorates further than expected, the European Central Bank said on Saturday.

In checks of the financial strength of the country's four main banks - National Bank of Greece, Piraeus, Alpha Bank and Eurobank - the ECB determined that even should the economy perform as forecast, the banks would need 4.391 billion euros.

The assessment looked at how many loans would go unpaid if the country's economy performs as expected up until 2017 - the so-called 'baseline'. It also simulated a 'stress' scenario, where the economy dips further.

(Reporting By John O'Donnell and Francesco Canepa; editing by Susan Thomas)