The operator of the Nasdaq Stock Market, Nasdaq OMX Group is planning to invest on strengthening its server security to prevent attack from hackers which is "under constant attack", the CEO of Nasdaq said.

Nasdaq OMX Group chief executive officer Robert Greifeld was quoted saying Reuters that there are people trying to slam into our system every day.

Hackers entered into Nasdaq’s server last year, leaving "suspicious files" on the exchange’s servers which led to investigation by the FBI.

The operator however said the trading platforms were not compromised, though some Internet-based client applications were vulnerable.

The exchange operator has been facing serious attacks from hacker groups counts itself among those, including Citigroup that have had recent cyber breaches.

Nasdaq, which runs stock and derivatives trading venues in the US and Nordic Europe, provides and stores sensitive market data.