A major cyber attack on the International Monetary Fund (IMF) sought to steal sensitive insider information, a cyber security expert? told Reuters?on Sunday.

"The IMF attack was clearly designed to infiltrate the IMF with the intention of gaining sensitive 'insider privileged information'," cyber security specialist Mohan Koo, who is also Managing Director, Dtex Systems (UK), told Reuters in London.

?The IMF and the U.S. Federal Bureau of Investigation?are investigating the cyber-attack on the Fund, the latest of a series of cyber?attacks on high-profile companies and institutions.

Tom Kellermann, a cybersecurity expert who has worked for the IMF and was in charge of cyberintelligence in the World Bank's treasury team said the intruders had aimed to install software that would give a nation state a "digital insider presence" on the IMF network, the Guardian reported.

That could yield a trove of non-public economic data used by the Fund to promote exchange rate stability, support balanced international trade and provide resources to remedy members' balance-of-payments crises, said Kellermann.

"It was a targeted attack,"? said Kellermann, adding that?the code used in the IMF incident was developed specifically for the attack on the institution.

The IMF insists that it remains "fully functional" while the FBI investigates the attack. But the Fund has declined to comment on the extent of the attack or the nature of the intruders' goal,?according to Reuter reports. ?

The World Bank said it had cut its network connection with the IMF out of caution, even though the information shared over the link was not sensitive, the Guardian reported.